I think a lot of people just stumble upon the forum haphazardly and join on a whim, so in many cases they aren't going to be very "committed" to the forum, if they stick around at all. Some people join and don't find much of what they expected, other people are simply fickle by nature and get bored with it easily. I'm sure a good deal of people are turned off by some posts they see on controversial issues and such. Then there is of course interpersonal drama, and some people find themselves too engrossed in the forum, almost like an addiction and feel the need to leave. I've also seen people leave because the social component overwhelmed them somewhat. And some are hesitant at joining to begin with. Many people browse these forums for long periods without joining from a sense of anxiety or an unwillingness to admit the scope of their loneliness, and then have second thoughts once they do join. And finally, with this being a loneliness forum, you would hope that eventually people will shed that state of loneliness and move on with their lives. So on some scale everyone's presence here is generally (and hopefully) transient in nature.
Edit: Also keep in mind the silent population of the forum. More than half the members of ALL have never even made a post. Who knows how they may come and go.