I'm not at the level required for MENSA (140 in the UK) but all I heard when I looked into it is it's a bunch of pretty dysfunctional people trying to one-up each other. I already have Reddit and HN for that :-D
Maybe being online turns anything into Reddit/HN.
On the other hand, word was most members are inactive and only a small percentage turn up at IRL events at all, i.e. strong (self) selection effects.
See if you can find some old usenet discussions about "prior art".
You don't need Mensa to meet some smart people, but it's an easy place to find them, for better or for worse (both of which can be interesting experiences, BTW).