All these social networks are just a load of shit if you ask me. Which you didn't, but I am going to share my half witted, un-thought out ideas anyway.
The concept behind all of them is exactly the same, the only thing they change is the look, its like a giant competition for rich people to see who can make the most professional looking website with some functionality.
The next big social network will be made by a nerd in their bedroom and it will have some technology that pushes Facebook and everyone else out of the game. I have a pretty good idea at what kind of technology is next but it is still 2-3 years away. Good luck with this shit, but seriously, social networking has gotten boring.