The Princeton Study: A Few Extrapolations
Facebook, with its billions of users, is at the top of the hill. As such, it's susceptible to the "next big thing" syndrome. Research out of Princeton predicts that it will be supplanted by 2017.
Basically, the idea is that Facebook is a meme--in the social-science sense, not the funny photo sense--that was passed around from person to person like a virus, and that people will eventually become immune to it. This is what happened to MySpace.
Geocities came under the influence of another entity (Yahoo!), but then they failed to adapt to the environment--cheap shared hosting with more features. People became immune.
Facebook mutates all the time. The only question is: do these changes benefit users? If not, the user's subconscious will create a mental "anti-body" to fight it. Facebook recently went public, and I think it's fair to conclude that their number one priority now is to make money for investors. This is not likely to generate changes that benefit users.
Using this model, we can say that Twitter has high adaptability to immunity, precisely because it doesn't change all that much and because it's not overly offensive to users. Similarly, Pinterest is in good shape because it is highly contagious and non-offensive.
We can further conclude that Google+ isn't likely the next big thing. People associate it with Facebook, and not in a good way. Besides, Google has its own tarnished reputation. The immune system is aware of it.
We can also conclude that all effective social media sites are somewhat offensive, because they have the potential to prove habit-forming.
The next big thing will utilize cutting-edge algorithms that allow people to find one another in new, and useful ways. It won't have an obvious monetization scheme early on, and it will be extremely easy to adopt. It will also incorporate mobile in some way, or may even be heavily dependent on the user's physical location. It will likely depend on the concept of the geo-fence to help people find things in their local environment that are of use to them--including other people.
The age of useless social media is probably drawing to a close. People realize now that social media is a time-suck.
So keep an eye out, and get in early.
Thoughts? Agree? disagree?
On the whole, you get what you pay for.