Mensa International Forums
The Mensa International website has a Forums section with discussions on a number of categories, which includes topics internal to Mensa as well as "real-world" subject matter. To access it, you need a member login, which is separate from the logins used in national Mensa websites. Usernames generally consist of the country code of the membership followed by a member number or ID from the national group; members worldwide can sign up for the site and gain access once their membership status is verified.
Activity varies by section with some topics getting more traffic than others, but participation is generally fairly light.