My rule of thumb.
If I find out anything about a WG, be it on FB, twitter or whatever, i keep it very much to myself.
I've seen loads of stuff about girls, it's quite difficult to be discrete when your pics are all over the internet with your tits out!
As above, you don't really know how the girl would take it, some might thank you, others might think you're a stalker.
I did stumble across some photos lifted from the PG of a very regular WG of mine on another site with a different name and location. I told her and she was very grateful, she e-mailed the guy and got them taken down.