I think you're right about narcissism being the motivation for a lot of the posts on those social media platforms, but I disagree with your stance. Particularly because I think blog posts are a significant source of technical information[1].

Those that are not, I don't find that they get in my way too much. If someone wants to keep what amounts to an online technical journal... search engines are pretty good at filtering chaff.

I do not participate in those social activities, but I don't consider it a great virtue that I do not. It would probably be good practice for writing documentation for my fellow developers.

[1] Cloudflare and randomascii's blog posts being some of my favorites; and I don't think the motivation is much different if at all.