godan verb (-ru)
become deserted; lose active users; depopulate
Slang, originally from computing/online communities. Describes a forum, game, or online space losing active members and becoming quiet. Can also be used for real-world depopulation in casual speech.
あの掲示板、最近過疎ってるね。
That forum has been really dead lately, hasn't it.
サービス終了が発表されてから、ゲーム内が一気に過疎った。
After the service shutdown was announced, the game world emptied out all at once.