My best friend has replaced me, what should I do?

So I've known my best friend for four years and we're inseparable. Her parents coach the bowling team and that's how I got started bowling. Now we've met this girl who's a year older than us and we've formed our little group of three best friends. Well they always go off and do things with just the two of them. And the stupid thing is either one or both of them will put it on Facebook. Just today our new friend put, "Going to see Gulliver's Travels with (my best friend's name)" It's really getting to me. I've never had a best friend for as long as I've had her and I still want to be her best friend. What should I do?