I think forums are a much better platform for expressing ideas and soaking up more detailed responses to give you better insight on what you posted.
Say, for instance, you post a selfie of yourself in a dress online. You'll probably get a lot of, "Yessss!", "That looks great on you!", "Fabolous!", "

", etc., replies. But do you get anything from that other than a cheap compliment?
If you did that on a forum, you probably wouldn't post it for the self-gratification compliments as demonstrated above, unless woven in, but rather longer form responses with substance, tips, and advice.
An example of this type of response on forums, which is completely different from social media, would be something like, "That dress looks great on you. You could probably put your hair up so that the neckline is more visible. A thin gold necklace would also look great with the earrings you're wearing, if you have one. I would probably wear this to a less formal event, so if you're planning on going to a wedding in it, you might want to reconsider."
The dopamine hit you get from a response like that on forums is probably why selfies aren't posted to forums. However, this is just an extreme example of what all social media posts give you versus what you can get from users on a forum.