well, i have one more idea for you: social network about people who are in social network and do not like the social network they are in or just simply hate social networks.
I think i should patent it.
oo fuck, wait a minute, it has been done already
http://www.hatebook.org/
http://www.isolatr.com/
http://www.snubster.com/
of fuuuck i am destroyed, there is mooore:
http://airbagindustries.com/introvertster/
http://www.enemybook.org/
http://www.ifuckinghateyou.com/
http://www.nosoproject.com/
shit... so as you can see you can make pretty much social network out of anything you can imagine, i will not start to talk about my next novel ideas of "people who like to eat carrots on wednesdays" and "loners who are so alone that they will even not find friends from facebook or myspace".
Give it a try! No dream is to ridicules if you have time left over & u can make good marketing - for sore you will get for starters some l337 hax0r n00bz to sing up who will want to share some keylogers whit everyone so they can 0wn the (hotmail)b0x etc or upload their sisters vagina pictures
Good luck and don't be bothered by small things like all the thousands of private & commercial forums, social-networks, portals and communitys
no really if you have some REALLY unique ideas u might be able to pull it off whit a lot of work and good webteam.. but do not just clone some crap.. build it and make it special