For those of you saying that someone will need "millions" to adequately start up a social networking site, that is definitely not true. There existed very popular social networks before the idea of Facebook and Twitter were even conceived, it is just what is currently dominant in the popularity category the same way MySpace was back in 2005 and prior to that.
If you are serious about making a venture out of this, I would encourage you to have a good idea and multiple ways to addict your users. Facebook did this well with having everything in one page at a time. You could be playing a game while chatting with three different friends and all the while check messages that were sent to your inbox.
With Twitter, they figured out that short sentence-long messages that can be repeated to other people to prove a point, emphasize something, popularize something, etc. can be very addicting.
The point is, having something original and, above all, addicting, is key to a social network's success. If it doesn't have these two qualities, it is doomed to fail, so don't waste your time if you want to try and copy Facebook and Twitter.
People really ought to do their research before commenting on something like this! Still, you can easily be the next Facebook or Twitter if you put in the right amount of time, effort, planning, incorporating ingenious, addictive elements, and a little bit of luck. Good luck!