Within my bubble of college aged people in the US in the late 2000s Facebook was already mainstream by 2007 (and I would say 2006, even, though I'm a bit more worried that I'm misremembering that).

But yes, I'm generally in the "it's the phones" camp of grumpy olds :)

Edit: Oh I just looked up the rollout schedule and it looks like it is more likely that I started my account in 2004. It might have taken it a year (at most) to become fully mainstream among my peers after that, so call it 2005 or 2006.