Someone who is constantly roaming from forum to forum, post to post, looking for new places to comment or threads to start. In extreme cases this can replace having a social life in the real world. The full on forum junkie can be spotted by their pale complextion due to lack of natural light, skitishness around real people and the fact that they hardly leave their room if they don't have to.
What's he doing?
Ah... - the usual, just browsing around his haunts, he's a forum junkie.