In an interview with Matt Haughey the $5 membership fee and advertising are discussed:

The $5 signup fee isn't subscription revenue [since it's a one-time thing]. It's mostly just putting a huge hurdle in front of having to deal with new users. 'Cause it's such a pain. The last ten years have shown that any time there's press, like the New York Times writes something about us, 300 people sign up and then wreak havoc for a while, and then go away. [Without barriers to entry] it would just be a nightmare.

Most of the revenue appears to come from ask metafilter since it is well placed for those seeking answers and services:

It's been really successful. I'm really lucky with the way Ask Metafilter works. Every question's about a topic that's easy to match ads to. The other weird thing is, ads aren't even shown to members. Members asking about what kind of digital cameras they should get want honest answers. But non-members searching for digital cameras online get [to Ask Metafilter] in that search.

Metafilter is one of the more interesting older websites on the internet. It has been around for a while and survived the early runs on trolling that younger folks and older folks that should know better do. It is interesting that it is successful in the manner it is.