It's nowhere near the size of those but I have run http://www.rubyflow.com/ for 10 years and it was basically a month of me posting several things a day and mentioning it on my then-popular Ruby blog before there was a tipping point where other contributors kept it going. I rarely post at all now.

On sites that require a social aspect rather than just link sharing, I imagine you really need to have a core group of people commit to engaging with each other before others are attracted towards it. I recall early HN being like that at least.