Yeah, that's how most XBLA games go (or even probably retail games in general). After the first few weeks (or less in many cases), people move onto other games.
As for online activity...I wouldn't get your hopes up. I don't know for certain but based on trying to get into lobbies for other old XBLA games, I'd say it's likely I'm correct. Another way to see is if there're a lot of "anyone play" threads or "anyone want to boost" threads.
As for the game itself - this game was widely regarded as a mediocre to awful game but I actually quite liked the trial, I just never bothered to unlock the full game for some reason.
The important thing is: if YOU like it, then buy it!
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