Facebook killers
Re: What will kill Facebook? Here are my thoughts. (FWIW, I posted this on Hacker News as well.)
The strong point of Facebook is that so many people are on there, including many who are normally less comfortable using computers. I assume it will keep growing for a while, as there is no real contender in sight. So let's say we reach the point where pretty much *everybody* has a Facebook account. At that point, it would only seem stronger than before.
The weak point of Facebook is that it lumps everybody into the same "friends" pool. This is not a new observation of course, but it *is* still a problem, and Facebook hasn't really done much to alleviate it. Sure, you can add people to different groups, set different filters, but for most people (especially those who are less tech-savvy) this is just *too much work* (or too difficult, even). So the problem remains, and as Facebook gets bigger, so does the problem.
I think that at some point, there will be huge opportunities for "spin-off" sites, that address some kind of niche. There would be a mini-social network were you talk to your family. Another one for friends (of the kind that you go to bars with, for example). One for networking. One for co-workers. Ones for specific games (quite a few people on Facebook have thousands of "friends", most of which they don't really know, but were added for the benefit of playing Mafia Wars, Farmville, etc). And so on.
Some of these already exist, like LinkedIn; and likely, the process is already underway. However, I think such sites would have more success if they were friendly with Facebook in some form or another; log in with your Facebook account, maybe share certain things, etc. (Look at Zynga; would their games have anywhere near the current number of users if they had not integrated with Facebook?)
When you meet new people, online or in real life, in whatever situation, the common denominator would be that they all have Facebook. (Still going by the assumption here that Facebook would get so big that people *without* an account would be rare.) So it would retain its function as a "hub" where you can find anybody at all; but the actual social interaction, in whatever form, would be done at the spin-off sites. You don't post work-related things to your Facebook because nobody cares but your co-workers. If you had a wild night out, you'd post pictures of it to a specific site, not to Facebook where your mom/kids/spouse/boss can see it. For those things, there would be the aforementioned specialized sites. Likely, it wouldn't be long before we would think of such a situation as normal; you won't post specialized info on Facebook, any more than you would post it in the newspaper.
I am just brainstorming here, but if that would actually happen, Facebook would slowly be supplanted by a myriad of smaller sites, each with their own purpose. Facebook itself would slowly become irrelevant other than as a hub to connect all those specialized sites. Like Microsoft nowadays, it would be big, bulky, probably rich, but it would just kinda sit there; the action and innovation would happen elsewhere.
tl;dr: Hypothetically, there might not be one Facebook killer, rather, it might be replaced by many smaller, specialized sites.