A slow down is not the end of the world, forum activity comes and goes, but it takes a lot to kill a forum. A dedicated team at the top will keep it alive, along with dedicated and loyal members.
I co-own and co-run
www.nzfortress.co.nz, which, in it's day (a bit before and during the beginning of my time) it was large for a New Zealand community. It has struggled in recent years due to a drop in the game that it was based around, but it survives, and since the new admin team has taken over, activity is the best it's been in several years.
No need to panic, i'm a new member, and i find myself loyal already. The board is very initimidating for new users, especially from smaller boards, and the best way for a good member to start is by being quiet, observing how things work and then beginning to contribute more and more, bit by bit.
There are also seasonal changes - people being on holiday or other such real world phenomena can boost up a forum's activity - thread:user ratios tend to go sky high during such times - what goes up must come down, and what may have happened is TFP is just coming off a really busy period, i can't say from being so outside of tfp though.
With your active member userbase and the amount of names presented as online, you've got nothing to worry about.