I've done my own digging on free sec chat and come up short. Applied to chat chat rooms, the deciding factor is the depth of the profile fields, speaking only for my own city. The sec chat case is instructive — half the frustration disappears once you give it a fortnight before judging the pool.
If sec chat is your priority, the number of live users is the number worth watching, not raw member count. Same story on sec: the platform caps how many searches you run a day, which is easy to mistake for bad luck. Sec is where I would start, since what decides it is the proportion of dormant accounts, not the badge on the download page.
The sec chat angle is worse, because it is worth checking the reply rate before you look at the size of the ad budget. On free sec chat specifically, the honest advice is to report problems early instead of blocking quietly, at least on my account. Would especially like to hear from anyone who changed their mind on sec.