I always wonder about that too. I've been on a few forums for several years now and its really conspicuous when a regular poster all of a sudden disappears. Sometimes they pop back in to give an update on what happened, sometimes they just disappear and never post another word.
On one forum I used to frequent, a regular got into a horrible car accident. His girlfriend hopped on his ID since she knew that he had several friends on the forum and kept everybody updated. Eventually he resumed his posting but he was never the same. Definitely weird.
Then there was the guy that was a long-time regular on a board who arranged a gear purchase or a gear swap of some sort, then totally disappeared with somebody's gear without paying for it. Its like he spent years establishing relationships just to con somebody out of a couple hundred dollars worth of gear. WTF? Now that's an elaborate, patient con! Then he had the nerve to show back up after a few years of absence, like nothing had ever happened. He didn't last long.