I'm not observing this at all. Most of my Millennial family and friends are on Instagram, regularly posting Stories about their family or travel. Many 30-year-olds+ are as hooked on short-form content (TikTok, YouTube shorts, Reels etc.) as the younger generation, but the latter are more likely to contribute rather than passively consume.

Messenger apps are very popular, with WhatsApp still being the number one choice.

I don't know anyone outside of tech who regularly posts on Twitter. Most ignore it completely.