GMail is mostly used for e-mail, which is something of a private conversation between (usually two) people. Facebook posts are quite the contrary: public, visible to the world at large, plus the users are actively encouraged to flag items they dislike. Now don't get me wrong: this is quite necessary for a social network - but using the same social network as the Universal Login For Everything is where this very feature becomes the SPOF.

Also, whereas GMail is not inclined to police the morality of its users, FB is. Not the same as data-mining, not the same as "legality" - for example, you have repeatedly used a word that FB retroactively considers a bannable offense, say goodbye to your account; or you have, a long time ago, posted pictures that are considered indecent under the new rules; or enough other users maliciously coordinate to flag an innocuous item of yours - and the banhammer falls automatically.

Moreover, GMail != e-mail: you are completely free to register with any of the bazillion e-mail providers, there is no need to use GMail; as for Facebook, there is only one.

I don't user Gmail either, partially because I just think the UI is terrible but primarily because I don't want another company controlling access to my email.