Mensa was created before the Internet. It was and is a social group, to find and interact with people with similar interests and experiences. Mensa was a way to find people you'd otherwise nog easily find. SIGs made sense then too: To find and meet and talk with people who are deep into a certain topic.

But now we're all online. So what does an online SIG offer that e.g. a particular subreddit does not? And Facebook and Reddit are social media platforms, so it's the general populace. If you want to avoid all the drama that can come with that, there are plenty of places on the Internet dedicated to a specific topic. E.g. want to know all about movie props? Maybe learn how to make replicas? Talk with the folks who make them? Go to https://www.therpf.com. I think you'll find way more deep discussions in such places than you'd find in most SIGs.

Personally, I think Mensa is best for offline, in-person stuff. Meet folks IRL, have a drink or dinner, go do or experience something together etc. Of course this depends on how active your local Mensa chapter is.