Well, since you mentioned that they might be really inviting your son, I'd ask him if he wants to go to them unless there's no way to go.
I'm not really sure why this annoys you more than an evite, since they usually seem to have similar levels of information to a Facebook invite. I've gotten more than one evite and had to go to my kids to find out whose friend this is, etc.
I've been using Facebook events for my kids' parties for a few years now, although that's mainly for family. It works well, and keeps the invite info in a spot they can find easily so I don't get five texts the morning of asking "What time are we supposed to be there, again?"
-Kris
DS (9/05)
DD (8/08)
DD (9/12)