Convenience. Facebook, for many people (myself somewhat included), has replaced email as the primary method of staying in touch. When email is only being used f

Convenience. Facebook, for many people (myself somewhat included), has replaced email as the primary method of staying in touch. When email is only being used for website signups and a few similar, non-primary tasks, merging it into what you use all the time makes a reasonable amount of sense. (Honestly, but not part of the point above, I don't have any issues trusting Facebook and Google with my data. I find it much less likely they will be hit with a Sony than others, and any really private data I have wouldn't go into any hosted service.)