There are multiple definitions for forum. The standard Internet definition is indeed "hang out, post lots of BS, troll newbs" - but the broader definition is simply "a venue for exchanging ideas", and SE certainly fits that.

We don't like to call it a forum, precisely because it brings to mind the former. But the latter (older) definition is perfectly applicable.

(I'm an employee of Stack Exchange, working in the "annoy gortok with pedantry" department)