Tumblr, Reddit and Twitter are community sites. They are driven by their users, with little or no overarching strategy beyond being next-generation BBS services that occasionally sell a few ads to keep the lights on.

Facebook is driven internally, from the top down, to accomplish murky, exploitative goals that are in the interests of nobody except the company and its shareholders.

That's why Facebook is worth hundreds of billions of dollars while Reddit can barely pay for their own bandwidth and Tumblr gets passed around between acquirers like it was a radioactive waste dump that runs a porn business on the side.

I don't know WTF Twitter is worth, if anything, but some people seem to think it's got a viable business model, so meh, whatever. They aren't Facebook.