I don't think they intended to grow slowly. When they launched, there were similar aggregators already (digg?). I think they just kept the site under the radar

I don't think they intended to grow slowly. When they launched, there were similar aggregators already (digg?). I think they just kept the site under the radar long enough to become sort of a Craigslist for content. The design was antiquated and simple, userbase was limited but loyal. It just caught on from there, sometime around 2012-2014 when people wanted more content per unit of time invested (vs other social networks).