What I can imagine is certain celebs visiting special interest message boards.
For example, I would be less than surprised to discover that Rod Stewart visits model railway enthusiasts’ message boards.
Not really…
What’s with the fixation on Bill Gates? If you’re looking to chat with him, he may respond to an email. Probably wouldn’t bother asking for money though. 
Harry Turtledove was a regular poster on the old AOL Harry Turtledove forum. S.M. Stirling and Michael Flynn also posted there.
For that matter, Stirling has posted on this board on a few occasions.
Considering what a model train wonk he is, I’d be more surprised if he DIDN’T visit model railway enthusiasts’ message boards. Same for Roger Daltry and Neil Young.
I kind of doubt that celebrities or high profile business people do “work” related stuff on the internet under assumed names; they have people to do that.
However, I wouldn’t be surprised if they’re on enthusiast boards for things that they’re interested in personally. For example, finding Alton Brown on a motorcycling board wouldn’t surprise me. Nor would finding Nick Offerman on a woodworking board, or Jay Leno on a classic car board.