In the online communities/groups I know, there have been a core group of people who are stable members (2 or more years). But I think you're right, TWB, online communities tend to be a bit more tenuous that f2f ones... in part because it's so easy to "physically" leave an online group. In some online communities, the core experience is the chat environment, so if you no longer show up to chat in real-time, people feel you have "left" even though you may still want to email with others. Perhaps showing up in real-time chat is considered more of a personal committment to the group because you "SHOW UP" and therefore are offering a more powerful "presence" in the group.