On a serious note, it happens, always.
Basically people get busy with a lot of things and they move on to other things.
I remember the era or Aadipa, Freestyler, Deleted empathy and Dexter - that was in 2004. More serious posts, people were friendly, no wars.
Next was Go4i, Sunnydiv, Goobi, Santhosh (Sansite), Pallav - the days of yahoo chat rooms, but some big fights (between some members) were there and finally most of them left. The era of portals - nukes !
And then the sr-ultimate, cool jeba - graphic specialists, blogs, funny and naughty comments.
Next I remember the OS war era lol. Gx, Arya, iMav and a lot of heated up discussions. MS vs Opensource, Apple vs MS and Praka was getting angry very easily ! People tried to provoke him and they could win very easily.
A lot of reasons behind the disappearance. Memebers get busy and do not get time to post in TDF. After a few yrs, members think they are mature enough and TDF is that of kids and N00bs and moves to serious kinda forums. Some start their own business, their own sites, their own forums.
Can't blame on mods or admin for each and everything. Moderation was not this much before. Members were behaving much better and mods were not under pressure, they were not having much job actually.
Some always stay, always help others and behaves in a very good manner (infra_red_dude for example), some pretend they are still young and appear once in a week or so to tell they are still alive (example : uncle bug err.. uncle Din). Some posts serious matters and prefer to stay away from all these kids play (example :Vaithy - a very senior and nice person). People differ, but for some TD Forum is a nostalgic thing, it makes them think of the golden days and in some cases, the memoirs bring them back to TDF, and the change always amazes them. New faces, new topics and new way of posting !
I think we need not worry on this, if we decide to help others, decide to be polite, decide to post something thats useful for the large majority and decide to share the knowledge, TD forum will be the best forum - as always.