Malky - I believe that some of the quietness may also be down to the change of scene (i.e. upgrade to PHPBB3). I'm sure there are some who are just a wee bit unsure about what to post, and how to adapt to the new environment. Periods of change are always strange things.
This is not meant to be derogatory in any way, but if you take away the sheer volume of posts that certain individuals or 'their cronies' were were making, then of course the number of posts is going to reduce. These are unlikely to be replaced overnight, or even within the first month or so. To get someone to a becoming a regular contributor takes a long time.
The facts are that two of the highest volume posters on the Forum have been 'lost' - likewise other high volume posters like Bill and Bobbie seem to have followed them into an almost 'self-imposed' exile, or at least self-imposed 'quietness'. I'm sure they have their own reasons for this? Personally, I don't think the forum is any less 'valuable' or applicable than it was 6 months ago, it is however slightly less 'cliquey', and as a consequence slightly less busy.
However maybe some of those who didn't post used certain people as an excuse for not posting, and just weren't into posting anyway?
The graveyard is full of 'indespensible' people - in my view the Forum will be here, in one form or another, long after we're all gone. I've always likened the forum to a Pub therefore it'll have it's ups and downs, quiet periods and busy periods - we are all merely passers-by in it's long an distinguished existence. I'm sure one day that you'll be telling your kids about all the old 'characters' on the Forum, away back in the beginning!
Viva le Forum

PS. Started writing this before I read Malky's response!