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.)

Why on earth would you trust PayPal/Mint/banks with money? Or Google with e-mail/search/browser data?

Why wouldn't you? What in specific are you afraid Facebook will do that Google would not?

why on earth would you trust anyone with anything?

not saying I'm retiring my gmail account, just that there are plenty of ways that it might make sense for certain cases, despite some folks' knee-jerk inclination to distrust this one particular company.

Because they are all a bunch of "dumb fucks" to use the words of the founder of Facebook