Are you friends with your coworkers?

My co-workers often go out together and I don't get invited. I'm pretty introverted but I thought we were all pretty friendly and we chat after hours and stuff like that. And I help them a lot in terms of helping them get clients, etc. It was something that bugged me last year, especially when I saw a list of their names on a piece of paper included with people who don't even work there anymore / partners of the coworkers.

I spoke to a friend of mine who said she was in the same situation at her work and it made me feel better.

However, today I see on Facebook my boss tagged everyone on a post about the New Year... everyone except me. I don't want to read too much into it, but I remember the last post he put up excluding someone, that someone was a person everyone dislikes and he was going to be fired and everyone noticed that he was the only one not tagged. So now I'm thinking why wasn't I included... does my boss not like me?

I try do a lot for the business in terms of marketing and stuff (even though they have a manager) but now I'm starting to feel REALLY alone and not part of the team.

Am I being stupid?