As said some years ago, the future is privacy, no spying on users. So when a user is already on facebook where the user knows, its a bad place, why should he/she start on another portal, that only reason is also only getting as much info on a user as possible. As said users , are not the dumb users as it was some 5 years ago, they are getting more concerned about what there data is used for. See example new internet explore adverting "PRIVACY".
Google is also not that popular as some years ago, specially among webmaster, but also the normal user, every time there is something about google its negative.