FB is insidious. When you 1st start out its all fun, you add loads of people, do the forums. After a while its horrid tho - people split off into groups, there are loads of "look at me me me and my life" photos, I've known people to start up affairs on there, seen friends fall out etc.
My friends list is minimal now, mostly people I also know in real life, and I won't have anyone on there who posts negative and unpleasant stuff, no-one over-boastful, no religious nuts etc. Im in 2 music-related forums, and thats it. Anything more would crowd my head too much and I don't want to see and know what people are up to on FB. People pretend its good for a catch up - but generally, it isn't. Its people sitting on their backside in their own homes "communicating" via a computer and pretending thats friendship, and only showing the parts of their life that look glam for FB. I prefer to pick up the phone, or text, and I see friends even if its once every 3 months for a few hours.
No, its not nice being excluded OP. & at least you know this "friend" isn't a friend. Not aiming to sound patronising but she shouldn't be so responsible for your happiness and good feelings anyway. If its mostly about your son well then you will have to get up and go out there look at other leads and options, with him in mind
meetup.com is good for mums with children who want to get out, aside from that there must be organisations who run groups that would be suitable for you and your boy. Use your laptop to get looking. SN doesn't have to be a barrier at all but if thats making you cautious then find a group you know will suit his needs. Just try it and see.