The part where it got a little shaky for me was the immediacy with which the lure worked to draw the man to her. I hadn't thought she had even given directions on how to find her before the phone cut off, and how would one even give directions to a swamp in a way that would actually be followed by anyone? "Hi, I'm a 1'm a 13-year old girl, I'd love to meet, follow 13th street until it dead ends, and then keep on going into the creepy swamp. You'll know you're in the right place because a monster will try to eat you." Any kind of meaningful directions would not only be weird and unlikely to be followed but also would be dependent on knowledge about the out-of-swamp world that Jenny doesn't seem to have. And even that would be dependent on the man living nearby enough that visiting the swamp was a real possibility.
I could be wrong, but I thought I heard her say "meet me at the millpond" (well established as a feature in the town) before the phone died, and in their conversation prior, he had volunteered that he lived in the town.
I'm sure she said to meet her at the millpond, and I think what's-his-name had said he lived a couple of towns over, so he probably would have been able to find the place.
By the way, I've definitely heard of Jenny Greenteeth before now, from at least two sources. I'm pretty sure they were both urban/modern fantasy. I think one of them mentioned her as belonging to the Unseelie Court or Winter Court of Faery, but the other treated her as more of a semi-dangerous naiad/nature spirit, unaffiliated with anything else. Over here in coastal North Carolina, U.S.A., I guess I'd tend to think of Jenny as rather like an alligator: dangerous if you're ignorant enough to get close, but not really innately evil -- scary but not soul-corrupting.
Like Unblinking, I liked the monster's perception of lanterns that were reflecting pools (which turned out to be smartphones), and also her speculation that people put their souls into them and carried them around with them now, instead of having their souls inside themselves. Very interesting!
The bit where she was trying to converse with people in the chatroom, and then parroted lines back in the private messages (I'm 13 too) to try to seem more human-normal, reminded me slightly of a recent Escape Pod, "Myspace: A Ghost Story," although in that case it was an AI.
I was relieved at how the story ended, because for a while, I was afraid Jenny was going to team up with the pedophile, or at least learn more from him (before killing him) about how to use technology to lure children.
In general, I liked this story quite a lot. Beyond the clever concepts already mentioned, I liked the explanation of how Jenny had adapted patiently through time, before the changes were just too much for her; the writing let me really sink into her viewpoint. I liked the pacing and the buildup, and although the ultimate ending surprised me, it felt right.