Oh Brighter, not another bloody thing! 
:(
How close is this friend? I would be very wary of talking about such things to a parent unless you know, like and trust her/him very well. My experience is that other parents are very ready to ignore their own DC's bad behaviour if they can find a way to blame someone else's - so they are much less likely to react with (e.g.) "My wicked son is taking drugs" than "My poor innocent son was sold drugs by your wicked one". You probably need to do something about your own son's behaviour, but you don't need him to be scapegoated for anyone else's.
I don't think you can ignore this. LSD (as you probably know) is a class A drug, so selling or giving it to someone else will always result in police action - at least a caution, and possibly even a prison sentence for 'supplying'.
You can't undo what he has already done, but you do need to be clear about what action you'll take if he does it again.
Of course one option is to call the police and/or throw him out now. I already know that you know that's not an easy option. :(
If you don't do that, you need to talk to him. Since it is going to be unpleasant to raise this with him however you broach it, I reckon that doing it straight out/directly is probably the quickest and least awful. I think I would simply say something like "I have reason to think you have been selling drugs, and you have done it in my house. That is NOT okay. If you ever, ever do that again, I will call the police and have you arrested/throw you out" (or whatever you decide your reaction would be - and remember you have to mean it and follow through :( ). Then if he denies it, you can say something like "Well,I'm glad you weren't so stupid, and now you know what would happen if you were". If he gets angry you can say "There's no point being angry. I'm just telling you what will happen if you ever do that (again)".
You don't directly need to mention FB, afaics... There are other ways you could have suspected or found out - from what you saw, or from the other boy's mother.
As for effects and risks, he'll probably be better informed that you are. It is worth saying (just to reassure you) that although LSD is a powerful psycho-active drug, it is actually one of the less dangerous ones around: Frank says "There's no evidence to suggest LSD does any long-term damage to the body or directly causes long-term psychological damage".
Good luck.