When Warhammer Online released, several TF2 regulars dropped TF2 to play WAR.
When Left 4 Dead released, it seemed almost all our regulars moved over to L4D--but have no fear! First-person shooter players tend to be fickle people and I'm sure we'll be back in droves when Valve releases another TF2 achievement pack (which I believe won't be until after the first of the new year).
As the novelty of L4D wears off, you'll see more players spend more time playing TF2. And with Mighty Gerbil planning TF2 game night events, there's a very good chance our former TF2 regulars will put down their shotguns (i.e. L4D) in favor of rocket launchers, flamethrowers, and mediguns.
In short: Have no fear! People will slowly filter back to TF2 in the weeks and months to come.
Speaking of which: We pay $30 a month for a server. Why do none (or nearly none) of our members play on the server we spend money on? I know no one wants to be the first player to join a server, but I've done it on more than a few occasions and found that once we get 4-6 players on, the server tends to fill out after that.