"the social network needs chips with screaming fast CPUs to deliver dynamic web pages and perform similar tasks"
No, not really. What they need is lots and lots of CPU capacity - but not individual speed-at-all-costs cores.
On the desktop/workstation, you can really only use a few cores for most things, so you need those cores to be as fast as possible. Intel are very good at that: the i7 and latest Xeons are packed full of really clever tricks to shave every microsecond off the execution time of individual instruction streams so you can get the next frame of your FPS drawn in time, or get that complex protein molecule calculated and drawn that tiny bit faster. Having more cores doesn't give you much benefit if any, for all the effort put in lately trying to make tasks multithreaded wherever possible. To double the speed, you can be quadrupling the cost - and if you need the speed, that's where you go.
Facebook, on the other hand, have millions of requests coming in each second. More and cheaper cores are a big win for them: half the performance for a quarter of the price means they can get twice as much throughput for the same money.
Intel see this too, of course: it's why they've been adapting their Atom core for server use, as well as portable. It'll be a tough fight, though: their biggest asset, Windows compatibility through x86, just doesn't apply to these big Linux server farms: a LAMP stack should be just as happy on ARM as on x86.
I wonder if/when Intel might get a big Atom sale to the likes of AWS or Azure? Or, conversely, when Microsoft might get round to offering ARM builds of Windows Server...