For Theory Of Geek most of my organic traffic is driven from social sites like Twitter and Reddit, Facebook visitors are actually much more engaged (average Reddit visitor views about 1.2 pages, average Facebook visitor 4.5 pages).
For CoderStack engagement (developers applying for jobs) roughly matches that of long-tail organic search traffic.
There is a huge difference between organic traffic and paid. The former has a huge selection bias, by definition, for those visitors that are interested in what you have to offer.
Ever tried doing a CPC campaign on Facebook for any of those sites? I'd bet that the usage metrics would come out way differently.
For CoderStack engagement (developers applying for jobs) roughly matches that of long-tail organic search traffic.