The online porn industry were the first people to really deal with scalability (of managing and serving web content, anyway), the first people to deal with online payments, the first people to come under really heavy, sustained hacking attempts, etc. Don't underestimate how much of what you take for granted technologically in 2012, they pioneered in the mid-90s.
A friend of mine built a lot of porn sites in the late 90s using DB2 and Oracle. He says that Microsoft's SQL Server folks came to him to talk about what it would take to get him (and the porn sites) to use SQL Server.
He said that he was confident that DB2 and Oracle could handle the load. They responded that SQL Server was much more capable than he thought and asked him about the workload.
Their response to his answer was "we can't even simulate that, maybe we're not ready".
It's not just now either, the porn industry was hugely influential in causing the adoption and spread of video (you know, on casette), movies and photography.
Indeed - I've heard it argued that the real reason that VHS won out over the technically superior Betamax format, was that Sony refused to license the Betamax technology to adult video vendors. Never dug into it, but it seems plausible.
Having just one deciding factor seems implausible. Straw that broke the camel's back? Sure. But calling it "the real reason" completely ignores every other relevant factor.
Social took off when Zuck tricked users into giving out their real names, when meant social networks grew much faster (as old school friends would connect).
Porn does "social", but only anonymous social, which is naturally handicapped.
If someone created a social network which allowed better privacy controls, the porn sites would love it.
