alt Hacker NewsFacebook only took off in a big way worldwide around 2010-2012 though (at least worldwide). I joined in 2009 (and long left) and I remember still having to explain to a lot of people what Facebook was.
But yeah, it's not a single point, there are many points around that time that are pivotal, like Google acquiring DoubleClick in 2008. GMail taking off around the same time and increasingly making blocking more and more other mail servers. Google and Facebook adopting XMPP and then killing off federation in 2013 and 2015 once they had a lot of users. Apple introducing the iPhone, which resulted in phones becoming the main consumption device for many people, in a very locked-down ecosystem for users, where companies can extract all the analytics they can get their hands on.
Also, smartphones made people terminally online, which strengthened network effects and made it more attractive to make social media and games addictive. That doesn't work so well if you can only access the net at night on the family computer that is shared with four people. Even though I was a student when smartphones came around, I'd only check e-mail in the morning and maybe e-mail and socials in the evening.
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.