The bubble I see bursting is this whole inline ad business. Virtually all of Facebook's and Google's revenue comes from ads. I don't know how many people click these ads and buy stuff, and obviously lots do, but I see the value in these ads dropping in price. If advertisers are not seeing a return on their advertising dollars, they'll go elsewhere.
Btw, I never use Facebook on my computer - only my phone, and there are no ads on the mobile app versions.