With one of those things I got so fed up that I build it with a friend, and sometimes when I see someone complaining about this problem that we have tried to solve and I tell them about my site every now and then I hear "Hum... that already exists in the US, that is nothing." all I can do is laugh.
Because I think that it doesn't matter at all if it already exists. I have build it because I enjoy building it I don't aim to be the next Twitter or Facebook even they didn't ( Orkut already existed BTW) and if it becomes a company or any other thing Great! otherwise I can point my finger at the screen and say "I build that from scratch" and that for me is worth a lot.