I think that a lot of the businesses advertise indirectly to the various forums (forii?). That is, a company like Canon or Nikon don’t necessarily directly purchase advertising on boards like Facebrook or tweeter - rather, they go through a third-party agency that uses some type of tracking/AI to push the ads out to specific demographics.
Post something about cameras on FB, and an hour later, you’ve got some Canon/Sony/Nikon ads in your stream of posts. It’s FB, Twitter, or the third-party entity that is setting up the ads based on what they think are your preferences.
To answer your question:
Technically, the plural of “forum” is “fora”, but that form has largely given way to “forums”.