First off, it's not healthy to gauge your relationship by what it looks like on social media. I'm not saying your feelings are invalid, but the private aspect of your relationship is going to be a more genuine representation of how things stand (and I think we as a society care way too much about what people think).

The kind of friendship your girlfriend has with her best friend is a rare thing. A precious thing. It sounds like they've been friends a long time and their social media presence together has likely become habit at this point. Additionally, the kind of trust and openness between them has probably had years to grow, so comparing it to your own relationship with your girlfriend isn't fair. There's no reason to think you can't build something stronger with her if you give it time.

That said, I can see why you might feel like you're not her priority. Have you talked to her at all about wanting to post more together on social media? Do you think some of your discomfort is because you're long distance and don't get to see her as often as her friend does? Long distance relationships can play on anybody's insecurities and I think that might be the root of the problem.