My 11-year-old son recently stayed over a friend's house. I found out a week later that his friend's 13-year-old sister also had two friends sleep over, which the parents did not inform me of at the time. My son confessed that the sister set him up with an email account and Facebook profile while they were all together on the computer. Should I confront the parents about not telling me they allowed girls to sleep overnight and about letting the kids be unsupervised on the computer? There are two things that I am against.
The questions that you raise are valid, but the answers depend upon the personalities involved. Obviously, this sleepover was not being closely monitored and thus featured the forbidden teenage shenanigans that characterize slumber parties. The question you will have to ponder is whether the slumber party was a destructive, shocking experience or a marvelous introduction into the world of adolescent foolishness for your son. If the host parents are your friends, you might give them some feedback about what took place under their roof. However, I am not sure you can criticize the parents for having their daughter's friends sleep over if you did not inquire about the sleepover circumstances beforehand. In any case, their daughter would be sleeping in her own home.
Also, it does not seem you can expect the parents to provide surveillance of their household computers during the sleepover unless you have specifically requested this beforehand. If you don't wish your son to use a computer without adult supervision, this is something that you should work out with him. Facebook has its own rules about the age of participants, which you might want to check out.
The issue underlying your query, I believe, has to do with shifting gears from supervising young children to teenagers, since both groups have a thousand opportunities for mischief if they lack the judgment to manage themselves wisely. By confessing, your son shows that he trusts you and values your input. I would meet him half-way by exploring his feelings, expectations, and reactions to the slumber party in a respectful fashion and support the growth of his own judgment. He'll be on his own very soon, both online and everywhere else.