First: I would wait and not bring this up right around a big party. It will put a damper on the occasion and could even cause a scene. As icky as this is, it doesn't yet seem urgent.
So, at a time when it could be brought up quietly, privately and in-person, with your partner and both FIL and MIL present, I would say, "I don't know if you know this, but some teenagers in the family and their friends have confided in me that they are feeling uncomfortable about some of FIL's online behavior. I agree with them that it feels a bit odd to have an adult who is a stranger to them following them on twitter or Facebook, or storing their photos on his personal devices. I can't tell you how to use the Internet, but I do think it would be wise to consider how you'd view another adult man interacting in the same way with DD online. Unfortunately, in today's world, your motivations don't matter very much, so there's no need to justify it. But I would like to ask you to stop."
If he lets you get out that whole spiel, it will do a few things: Build him up as the responsible adult, take away any claim that you are accusing him directly, and put the onus on him to act. It avoids directly telling who "told on" him or telling him what YOU think of his behavior and allows him to reconsider without going on the defensive.
Personally, this sounds creepy and uncomfortable to talk about. You have my best wishes!