[shameless bragging mode]I’ve been mentioned by name in books by Newman and Turtledove.[/shameless bragging mode]
A surprising amount of SF authors used to hand out on the AOL SF book boards. Harry Turtledove’s was at one time one of the most active boards on AOL. Turtledove, Stirling, and Flynn were among the daily posters. Obviously there were some top notch discussions of alternate history. However Harry eventually decided that his online activities were taking too much time and energy away from his professional writing and cut himself off. After his departure the traffic gradually faded. I myself no longer post on AOL.
Kress and Newman also posted on their AOL boards which is where I met them. Wrede is a personal friend of Lois McMaster Bujold and was (is?) a frequent poster on her board.
I once emailed and got a response from Dave Abrruzezzese (sp?), former Pearl Jam drummer. He had a message board for some reason, I forget why, and I forget why it was that I emailed him. Oh well.
I’ve gotten personal responses from authors Julia Quinn and Kate Forsyth (I think it was Kate Forsyth). I think I’ve also received a personal email from Elaine Cunningham, but it was a long time ago, and I can’t remember for sure. Julia Quinn is finding it more and more difficult to respond as she’s becoming more famous, so I try not to bother her, but she’s responded to every email I’ve sent her.
Even more famous, even though it’s not really online, but if you send Sue Grafton a letter, she responds personally (it’s not a vague form letter, but references things you mention in your own letter). I suppose it might really be an assistant writing the letter, but it’s at least signed in pen by her. And she sends out Christmas greetings signed by her.
When Patti D’Arbanville was on Guiding Light she used to chat with her fans all the time and regularly posted to message boards. She was very cool and acted just like any other poster, which eventually got her into trouble. She got into a big flame war with some fans who didn’t like her character and ended up disappering from the net. She’s probably still out there somewhere under an alias.
Not really celebrities, I guess, but I was pretty involved in the community for the game Sin, developed by Ritual Entertainment, including developing mods for the game. I was in correspondence with several of the developers, but just about everyone in the community had talked to them at some point.
Richard Herring of BBC’s ‘Fist of Fun’ and ‘This Morning With Richard Not judy’ posts often enough in the comedy forums at NOTBBC.com . There are other semi-famous people who have jobs in ‘the biz’ who have posted there also.