If you remove FB and Reddit from the list you can definitely see a larger number of queries for "forum" over "forums" on the trends. It's not a lot of traffic but it's significantly higher for "forum".

If you're searching for the extremely generic term "forum" I can only assume you are doing dictionary lookup?

Would be more interesting to compare searches for the names of several forums (not just FB and reddit) and see how the category has evolved over time.