There's a girl I like, and when I say like, I mean REALLY like. Me and her are kind of friends, i.e. were not really close, but we talk and get along well.

The person who introduced us is a friend of mine, we've been friends for a while now (2 or 3 years), and he's a really nice guy.
Recently they've started dating and their really close. Normally I wouldn't mind, but I see them together a lot, both when were out and in Facebook pictures, and it makes me feel really uncomfortable and sometimes kind of sad.

So far I've told no one how I feel, and I don't intend to.
I just need some advice on what to do.
Thanks.

Well they are both individuals. If you like her then you should have told her. Your friend has no fault in it. The girl choose to date him. So don't let come this into your friendship.don't be sad. They have right to date anybody they wish.