YCombinator was a successful startup accelerator, then built HN.
The folks behind Stack Overflow ran a popular blog (Coding Horror)
In those cases they had a smarter-than-average population that had some shared interest (e.g. "COMM-unity".)
When those sites came around, Google was recognizable as what it is today, but Facebook was not. Hyperlinking wasn't seen as a crime back then, and Google didn't see organic search results as competition for paid advertising.
Circa 2000 I helped someone build an online community of 400,000 in Brazil -- the start of that was sending 10,000 spam emails, which got us 2,000 sign ups (as incredible as that sounds today.)
Pay attention to retention. If you think it is a hard to get people, then it is all the more important to retain people and to think about the path of getting them to contribute.
I think a lot of people who want to try marketing don't understand how much work it takes. If I wanted to advertise a concert at a college campus I'd expect to put up a (8.5x11) poster for every 10 students or so. I see a lot of people print 1 poster per 1000 students and call it a day.
Some people don't want to make extra designs and wait for the printer, other people don't want to spend the $, other people don't want to walk to every building on campus.
You will hear stories of someone who got an exceptional break (that 20% effective e-mail blast is one) and wish you could get one. Don't let that wishing get in the way of doing the hard work, in fact often it seems you get the "break" by accident when you are doing the "average" work.
Avoid the Girardian ("mimetic") traps that are popular in many places. In particular, do not be "part of a herd" without a well-examined understanding of why your actions benefit you (incl. 'your brain thinks that cocaine raises your utility function but it doesn't)
Specifically, those "Like us on Facebook" buttons feed data and traffic back to Facebook from hundreds of sites. The ratio of engagement the world gets from those buttons is vastly less than the engagement that the world gets.
If I was talking to customers on the phone and trying to get them to buy into a bad deal like that I'd have a hard time, but when people see that "everybody else is doing it", it's hard to get people to think the consequences through.
Those sites you mention all predate the Facebook age. To linear order, "new communities are not being created". If you look closer, new communities are being created but they are smaller and separated from the social media lamestream.