Actually, pretty much all facts are actually just opinion. e.g. it is my opinion that the earth is a globe, though opinions vary. I confess that one site I've frequented had a hard and fast rule: If you did not state something as an opinion you were required to post a link to an internet site that supported your statement. It was sort of anal, IMO. Some of us tried to include a sig line that said, basically, "The above post is opinion." Nope. they forced us to hand type it in every post that didn't have a link to a source on pretty much anything. It got pretty funny.
I just cut posters slack on anonymous forums. I assume every post is opinion. After all, none of our posts are "News articles".
Of course, all of the above is just my opinion.