When Facebook actually needs to live up to the fact that continuous VC-funding isn't a viable income stream, the bubble will burst.
You need to make money to be successful, always have and always will. Like the first time, the ones that can do that will move into the next round, the one's who can't, won't.
My bet is Twitter. Its ubiquitous, and yet: useless. It's userbase is relatively small, and yet there's no good way to monetize them. Its provocative enough that people either love it or hate it, but not nearly useful that people will get an account just to have one (like richer social networks, where people joined Myspace/Facebook to avoid network isolation).
I don't think you can say the same for facebook. Very few people will pay for this thing. I'm willing to bet that their own internal research confirms this. Facebook's biggest issue with regards to profitability is not so much that they need to find ways to make money, but that they need to find ways to make enough money to pay back all the money they've already used. They are leveraged to a crazy degree, and seem to spend like drunken sailors. They've got a huge workforce and have innovated little since launching their "platform", which has been all but abandoned - from a user perspective - because of abuse.
They've got a leadership problem in my opinion. And that's a hard problem to solve.
Both Facebook and Twitter would collapse if they charged. Not as many people would pay for Twitter as you seem to think.
Facebook's users at least see Ads. I have actually bought stuff off Facebook because the targeting was perfect.
And Twitter's user base is about 6 million, which is also a far cry from Facebook's 50 to 175 million, depending on what number you believe.
Where are they going to go? Back to email, IRC, or Blogspot? I doubt it. To Twitter's cheaper competitors? How are they going to stay in business?
Rather than just die, Twitter will monetize. Though I doubt that they would monetize by simply charging people to read Tweets. That's much too blunt. There are more subtle methods of throttling the free-account users and thereby encouraging them to pay: limit the outgoing tweet rate, limit the rate of direct messaging, limit the number of followers or followees, lower the number of allowed characters by five for each successive tweet that you write on a given day...
Here's an idea: charge for low latency. If you're on the free plan at Twitter you can make as many tweets as you like and they will be sent to your followers and posted to the public site... eventually. Within, say, 2 hours (or 4 hours, or 8 hours). But if you want instant Tweeting you've got to buy a (fairly cheap) subscription.
Any such change will cause an uproar, so Twitter won't do it unless they have to. But if they have to, I expect they will. They're not just going to fold up and die.
I actually don't use Twitter at all because for me Tumblr does far more, and it does it more effectively. (Tumblr actually has a Twitter client now.) There are a lot of other companies that do what Twitter does. Friendfeed, Jaiku are the two that come fastest to mind.
I'm certain Twitter won't die, because there are a lot of options open to them. Monetization directly is not their best option, though. They've been free for too long for that to easily work.
Where do they go: facebook, orkut, linkedin or any other social network for 'status updates'. Some free service for 'realtime news'. There's no business secret or technical trickery on twitter - it's really a simple and reproducible service...
On how to survive: by using VC money for years, maybe? Expecting to get bought by GOOG (oops, I think jaiku already did this)? Not making any money at all (exactly what twitter does today)?
That would be the perfect moment for someone to launch a similar tool. I guess Pownce's history would be a lot different if it could be launched the day twitter decides to do that.
seriously, $10 a month to read what your friends are doing? As useful as twitter can be, it's never worth that much a month!
Twitter is monetizable - it doesn't need to be "useful" in any productive sense. Flower shops are still in business but they just flowers. Jewelry stores are still in business and they just sell pretty rocks.