Product

Launch dates for broadcast social networks with at-scale (100M+ users) retained user bases.

YouTube — 2005
Twitter — 2006
Instagram — 2010
Pinterest — 2010
Snap — 2011 
Telegram — 2013
TikTok — 2016
Threads — 2023

If you remove Threads (Instagram drives distribution, so derivative of that graph) and TikTok (billions of dollars of paid acquisition + ability to plug into a mature ML model from Bytedance), the last sustained broadcast network created by a startup was Telegram (and it's mostly messaging).

11 year drought.

are you excluding reddit and discord because they're not 'broadcast networks'?

Reddit — 2005

But not really a user social graph