HM666 recommended a text bubble for the first one. But I was thinking the circle works well, if it went a step further and was part of a basic person icon. Since the euphemism we use for forums is "community", then a group of people better represents that. I know, you run into the issue of scalability and it looking too much like a messenger icon.
Then again, vBulletin has typically chosen that type of icon to symbolize an active community.