My friends never want to hang out?

[Kelsey]

I have a small friendship group of around 8 they're my only friends unfortunately. I always used to think that my friends were just anti social but it turns out they hang out with other people.
Two of my friends live opposite each other so they are constantly over each other's houses and never out with anyone else.
My other three friends both have outside of school friends that they will hang around with.
My last friend hangs around with people I like as well outside of school and invite others in my friendship group but not me.

It doesn't really bother me until my friends are posting pictures of them on Facebook going somewhere that I wasn't invited to with other people.

It just seems like they don't like to hang around with me. Two of my friends will happily share their previous weekend plans with me and all I do on weekends is sit at home alone.
I don't know what to do. I'm very shy so making new friends is hard for me and I'm not into sports. Help?!