[fragmede]

forum? Instagram is the mainstream product you're looking for, and the text rendering there is decidedly GUI based and not markdown-esque

[lmm]

Instagram does not offer "highlight some text, and click the Bold button, or maybe Ctrl+B". The normal way to put bold text on Instagram is to use an online "bold text generator" and copy-paste from there. I struggle to imagine that it's that way because people like doing that.

From a quick search only "mainstream social media" which offers native bold functionality is WhatsApp. And it uses markdown-style.