Because you can leave Google. You may not want to, and they do a great job of having features nobody else has, but if they piss you off enough, there's plenty of email providers.
Facebook, on the other hand, holds your social network hostage. We'll always resent any site that takes advantage of that.
I think you are overdramatizing Facebook's importance quite a little bit here. It's not like your friends will hate you when you quit and I have seen plenty of people quit online social networks while retaining their life just fine.
I could make up a similar argument the other way around: to me, services like Facebook provide little to no value; I can quit them just fine. I have communication details of a lifetime in Google, I store my documents with them, organize my calendar. (Note: I have heard Google's export features are quite okayish so that argument does not really hold.)
I think you're underestimating Facebook's social importance. Sure, your friends won't hate you, it won't be the end of the world, but there is a palpable sense of disconnect.
And there are more people who rely on Facebook for social connection, than people who rely on Google for business and finance (especially if you're talking about the under-25 crowd).
I believe your claims but is there any evidence to back it up (and is that possible on fuzzy metrics such as palpable sense of disconnect)?
From your other comments I understand you are from the US and I guess its a wholly different matter across the pond, but I for one cannot reproduce these effects here in Germany. Sure, most -- rough estimate: all -- of my contacts do have accounts on various social networks (me included) but we have never critically relied on it. It never gained quite as much traction as I always hear from Facebook.
Facebook, on the other hand, holds your social network hostage. We'll always resent any site that takes advantage of that.