I wish this thread were labeled Do you believe an actual Time Traveler would bother visiting a forum?; it would frame my question in a more reasonable context.
Now, I don't doubt that a time traveler COULD come back and visit a forum. Time travel is an accepted part of our current understanding of physics, and I'm sure it will be possible to travel through the 4th dimension at some point in the future - even if that happens to be thousands of years from now.
My question is would they bother spending time on the internet? Especially when the experience is going to be exponentially clunker than anything they're accustomed to. Look at cell phones - earlier models were the size of a brick or required a suitcase to carry it around... And that's only 20 years ago. We're now entering the era of wearable computers, like Google Glass. What kind of interfaces are we going to be using 100 years from now, and will it be of any benefit for them to travel all that way just to visit Yahoo GeoCities?
We've got the Internet Archive, man. At most they'll probably just download it all into their Matrix skull socket and spend their resources marveling at the tragedy 9/11 or the triumph of watching us launch the first Apollo rockets. Playing on the internet pales in comparison to what's going to be available for them to experience.
If all my previous arguments fail and someone REALLY wants to come from the far future to visit a site like GLP, ATS, Unexplained-Mysteries or Paranormalis, it'll be to wonder at how backwards we are for believing the things we do. It would be of no benefit to anyone to reveal themselves, let alone post and risk changing something or having the black helicopters light you up.
I think the idea of someone with the ability to time travel being remotely interested in a community forum is pretty narcissistic... There are far more interesting things in all of time and space than Earth's internet forums...