pmrl, that's exactly how I feel. In two years, he'll be totally on his own, so you have to loosen those apron strings at some point. He's never given us a moment's worry, so I trust him. The fact that he let me see the FB invite says there is a level of trust from him, and I hate to blow it by overreacting.
He went to a slightly smaller party a few weeks ago with some of the same kids. The difference for me was that I knew the parents well, had been to their house to socialize, worked on the same school committees, etc. Interestingly, he said he didn't enjoy the party that much and even called and asked me to pick him up early. These kids are kind of A-list kids, popular, hard-working and some are kind of preppy (as preppy as his alternative school gets!). He said he didn't know any of the music they played (indie, but he's into classic rock).
This guest list is a little larger and includes more friends who are in his usual social circle. Those are the kids I know well and don't worry about!