Where did my friends go?

I'm grateful for the support of friends in virtual spaces, but have heard nothing from the people I've considered good friends for the past 10+ years. No texts, DMs, phone calls, letters/cards, nothing.

It's even odd to think that there were no offers of emotional support at all when I made a statement about my dad's death on social media.

Why does this happen? I know I'm not the only one.