Legally, yes, Twitter is. By legal standards, a monopoly does not have to have 100% market share. Your ignorance is not an argument.
“Second of all, does that mean the government can regulate and tell businesses what to do?”
If those businesses are getting funding & special protections from lawsuits by the government, yes. If they want to be publishers with editorial control, not neutral platforms, Facebook, Twitter, etc. should give up section 230 immunity.
“And third of all, why dont you just pull yourself up by the bootstraps and invent your own social media app, capitalism breeds innovation right”
Because when we try, scum like you get other monopolies to interfere or shut us down, see Parler, see Gab. Your ignorance is not an argument.
Do you ever get tired of being wrong all the time?