T.S.
Well, it's not parental alienation (by any stretch of the imagination) but it is pretty jerky and immature.
Try to take a breath. This man is NOT hugely influential in your children's lives. Whatever he tells them this weekend they have a lifetime with you to counteract.
Remember that you want what's best for your children which has nothing to do with what is good or bad for their father. If they want to see him this weekend (which is sounds like they do), it's best to let them go. There really is no advantage he can take other than in his own ego.
My advice is that the next time they come home telling you what stressful thing their dad piled on, just say something like "Oh sweety, I don't want you to worry about that at all. You know that Mommy is taking care of it. The grown ups will make sure that all of that gets handled, children do not need to be in the middle of it."
As for the Facebook page, you have legal custody. You and you alone decide what and when they can sign up for stuff. Delete the account and tell your kids that Facebook is not for 9 year olds. Period. Who cares if you're the mean one. You're the one in charge.
HTH