By John D. Sutter, CNN

(CNN) – Facebook is trying to be the “forever network.”

When the company’s hoodie-wearing CEO, Mark Zuckerberg, announced a new feature called Timeline in September, he proclaimed that Facebook would be the website – or social network or app or whatever – to catalogue life from birth to death. The site even created a place for users to upload their baby photos, to signify the start of their Facebook lives.

This, of course, has happened in Internet history before. There was a time when tech pundits thought MySpace, Friendster and AltaVista would be around (and relevant) forever, too. But what’s strange about Facebook’s audacious birth-to-death claim is that, to many people, it didn’t seem all that strange.

Maybe Facebook can last forever. Maybe, 901 million users later, it truly is something different.

As Facebook goes public, one question the non-Wall Street crowd should ask is this: Can anything ever replace Facebook? And, if so, what might that look like? If not, what could even compete with Facebook at this point?

Some writers, including Alexis Madrigal at The Atlantic, have argued it’s high time for the post-Facebook era to emerge. Plenty people seem to agree with that. A recent poll from the Associated Press and CNBC found half of Americans think Facebook is a “passing fad,” according to Time’s tech blog.

That may all be wishful thinking, though. More likely, we’re coming to a time when apps and micro-social-networks don’t dislodge Facebook but at least compete with it in some respects – or even augment its capabilities. The scrappy photo-sharing app Instagram did that. So did Glancee, a “social discovery” mobile app. But, obviously, Facebook bought both of those.

CNNMoney: 7 startups Facebook should buy

Here’s a list of 9 sites and apps that kinda-maybe-sorta could challenge Facebook’s dominance. Or, at the very least, offer a partial alternative. Check them out and feel free to fight with us about the list in the comments. (Note: User numbers vary by site and app; it’s an apples-oranges thing).

Highlight (number of users unpublished): This “social discovery” app was the buzz at this year’s South by Southwest Interactive, a conference in Austin, Texas, that makes or breaks many tech start-ups. Essentially, the app aims to give people real-time information about the people all around them. “San Francisco is a city of 800,000 strangers,” Highlight founder Paul Davison told Time. “You sit on the bus next to each other. You stand in line next to each other. You go to bars and meetups to meet each other. You walk by each other on the street. And you don’t know anything about anyone you see.” Highlight gives users a “sixth sense” about what these people all around you like and dislike, and whether you might be friends.

Path (3 million users) Founded by ex-Facebooker Dave Morin, Path has a couple things going for it that Facebook doesn’t: It’s mobile-first, which is important in a world where people tend to network on their phones more and more than on their desktop computers; and it’s intimate. Path caps users’ friend lists at 50 people, ensuring that you’re actually communicating as the real you with people who you really know in real life. “Path is really focused on close friends and family, and developing deeper and more intimate relationships between the 50 closest people in your life,” Morin told CNN last year. An app redesign won Path a new wave of support from the early-adopting tech public, but a privacy snafu in February, during which it was revealed that Path stored users’ phone contact lists, may have eroded the trust of some people. Morin apologized for that data slip, saying it was accidental and had been remedied.

Pinterest (11.5 million monthly active users connected through Facebook, according to AppData) A recent report from the marketing firm Experian declared Pinterest “the hottest social media start-up since Facebook and YouTube.” The site lets users “pin” photos of their favorite Internet findings onto personal boards, which can be shared with friends or kept private. Pinterest actually launched in March 2010, without much notice from the tech taste makers in Silicon Valley. It surged in popularity in late 2011 and early this year and, according to Experian’s report, is the third most-visited social networking site in the United States, excluding mobile usage. The site is especially popular with people who are design- and art-minded, or who are trying to plan events or decorate a new house. Also: President Obama uses it. But there have been questions of late about whether the site’s popularity is waning.

Viddy (36 million monthly active users connected to Facebook, according to AppData) Billed by Paris Hilton as “like Instagram, but it’s video,” Viddy lets users record and upload 15-second video clips, then stylize them with retro-hipster-looking filters. Viddy isn’t the first to try the short-video-app thing. An app called 12-seconds pioneered this space a couple of years ago and then shut down. But the startup seems poised to capitalize on Instagram’s success as an app that can make anyone’s photos interesting and fun to look at. Maybe Viddy can do the same for short video messages.

Tumblr (56 million blogs) Tumblr is more of a blog network than a social network, but it is an increasingly popular platform for creative types to share videos and short stories about their lives. Founder David Karp describes it this way: “Tumblr is the best place in the world for some of the most creative people in the world, so we’re trying to build basically the best tools for self-expression.”

Google+ (90 million accounts) In terms of format, Google’s social network is the closest thing to Facebook. But it’s not nearly as successful. The network, launched last summer and immediately seen as a latecomer follow-up to Facebook, is the constant butt of jokes in the tech world. Want to put a photo where no one will see it? Post it to Google+. The funny thing is that the network continues to maintain some steam and relevance long after the haters thought it would die. In part, that’s because of the big-fisted control Google maintains over Internet searches. Since items that are posted on Google+ are given some degree of priority (there’s a big fight about how much) over other content, some people use Google+ just so that their digital lives are more searchable to the public.

Twitter (140 million active users) I mean, you know what Twitter is by now, right? It’s been six years. The micro-blogging site has turned into more of a news feed than a social network these days, although it’s still used as a one-line public chat program by lots of people. As Rebecca Rosen notes at The Atlantic, there are websites (and lots of tablet apps) that seek to aggregate and display links shared over Twitter. It’s “Twitter without the Twitter feed,” as she says, and that may well be the future of this network, which pioneered real-time communication online.

LinkedIn (161 million accounts) LinkedIn is often overlooked in the debate about online “social networking” sites. Mostly because it’s no fun to socialize when you’re wearing a tie and shoving your resume in the face of everyone you meet. But the 10-year-old site (that’s two years older than Facebook) has proved itself as the premier location for a special kind of networking – the business-y kind. And there are some signs it’s loosening up. The slickly designed iPad and iPhone apps for LinkedIn get rave reviews from the tech set. And, anecdotally, more people seem to be posting about their personal lives on a network that was designed first and foremost for professional connections.

Bonus edition: Airtime (0 users) An app with no users? Why’s it on the list? Remember that dude who Justin Timberlake played in “The Social Network?” That guy’s name is Sean Parker and he’s launching this much-anticipated app on June 5. The Napster creator is regarded as one Silicon Valley’s main taste makers – or at least one of the richest. So it’s possible this social-video startup could rise to the point that it challenges Facebook, which is a company Parker also invested in. Who knows? But it’s worth watching.