A great way to find even more is to look at the groups and pages each of the above are following. I especially like the first 5 on the above list. Just so you know, there are tons of great pet rat pages on Tumblr, too.
Looking forward to seeing what others recommend here! Thanks for asking such a great question!
Rattie Tattie Rescue and Real Rat Lovers Want To Know! are the only two worth adding. The rest suck. I'm sure some other members will agree with me and tell you some of the things the other groups have done to them.
I am currently a part of two.
The rat fan club, and rats are awesome.
Personally I highly dislike rats are awesome, and much prefer the rat fan club.
Mostly because of the mods, and the general feel of the members.
There are a ton of FB rat groups, and most of them are awful. Rattie Chattie and Rescued Pet Rat Owners are decent. RFC and Rats are Awesome are huge so there are a lot of idiots and drama and encouraging of bad behavior, and I'm not a fan of their moderatorship tactics.