What decided it for me with Zoosk was how many profiles logged in this week.
Out of roughly 50 matches on Zoosk, 7 turned into an actual conversation.
If you care about how much of the base is genuinely looking, that alone should decide whether it is worth it.
My rule for the app now is simple: check whether it thins out after the first week on day one.
The signals that told me most about Zoosk:
The closest alternative I found was Luvdate.