Wait a minute; it sounds like they were setting up going out, etc. and I don't see why you didn't post and say you wanted to go too? Going out and doing things with others/a group is usually very informal and just sort of "happens" and expecting to automatically have people invite one is not good communications.

I notice one of the people is your sister; are these friends the same age as you, older/younger? If you want to be with certain people, you have to let them know, you have to include yourself; that's your "job". No one else, not friends, relatives, therapists, parents, etc. can look out for you and your life as well as you can/should.

Next time, post on your friend's page something like, "Hey, me too! You owe me a drink/meal/ice cream (whatever sort of place they're going to) for the dog-sitting!" Or, plan something yourself and invite them this weekend?