I'm still a newbie, after one year. I'm an old hand at internet forums, though :). Since the days of usenet.
Forums like this, that are specific to a certain topic, do suffer from repetition. And long time members do reach a point where there is very little that is new for them. They may end up leaving the forum but staying in touch with their forum friends elsewhere (facebook/email/real life).
The other kind of forum one that is more open to all topics under the sun. But even those get stale if there is not a continued influx of new members, and an openness to new perspectives.