There's an entire category of web-applications derived from the original forum idea. However, I'd say, while fora were once a frequent thing (and, considering Steward Brand's the WELL or FirstClass, maybe the original internet application), they've become rather rare and there are only just a few "community watering holes" left.

P.S.: Here's an exemplary link to what was once the major FirstClass node in my country, which first migrated to the web and then became obsolescenced [is this a word?] by Facebook (the tagline reads "forum for politics and society", operational 11.11.1992 - 1.12.2012): http://blackbox.at