In 2011 over a 2-3 week period, three large Diaspora servers shut down and took out roughly half the known fediverse. Diaspora provided account export but no import facility. Most of those people returned to Facebook and never came back.
Coincidentally this is when I began work on Nomadic Identity, a theme you'll hear me repeat over and over -- even though I get laughed at and ridiculed by pretty much everybody; and even though we've been successfully saving peoples' asses from random server shutdowns for a dozen years now.