Sometimes people join forums, but have a lot going on and don't go back for some time, only to find they've forgotten their password. Ha ha, at least that is what happened to me after I joined the sister site to this one (the garden forum).
If there is concern about cleaning up any unused user names, perhaps e-mails could be sent to those who haven't posted with the option to quit membership, choose a new password, or just verify that the username is still in use.
Those of you who are new to the forum, I think it just takes time for everyone to get a feel for other people's personalities. Similar to being the new kid at school, but maybe a little more pronounced because there are many many members and we aren't all in the same place at the same time every day. The more often you post, the more others will get a feel for who you are and respond back.