When Facebook started, they bootstrapped networks by downloading & matching contacts from email. Twitter, and everyone else, did the same.
Threads is blowing up because Facebook is using their monopoly on the social graph. Legislation to guarantee easy, fast access to your own contact lists for use in non-billionaire-owned media would help level the playing field, because Zuck & co sure aren't going to give the connections they stole back to us otherwise.
@blaine I think threads is going to be the next Google Social (or whatever it was called).