It's basically the same what you use on Facebook. You post short stuff about yourself, friends, what's going on right now, what you are thinking about, post or share links, articles you have read somewhere, shopping, clubbing, fishing, etc. Basically just updating your status. reaching out, telling people what's happening with you, or what's going on in general. It can be one word or a whole paragraph.
The major difference in Main Feed vs Forums and Blogging:
It's fast paced. That's what everyone sees first, you get responds (likes, shares, comments) fast, and the best way to reach out, let others know what's up at the moment. It's visible on main feed for a shorter time.