Another thing email doesn't do well: the idea of "I have something to say, but passively". Maybe you want to announce a change in your life, or a party, or some thought that you had. It's far less annoying to put it on Facebook where your friends can digest it (or not) at their leisure, rather than in an email demanding to be read.
"Hey, what's your new address?"
"I emailed it to you, go search through your messages"
The only problem I see with your email sentence is that some clients lack a decent search functionality.
Some improvements and it would IMHO be as easy, if not easier, as opening Facebook, trying to find a contact and fight against the timeline to find the information you are looking for.
I think the party part is a bit of personal choice. If I run a party I want to notify $foo people. And $foo is always a smaller number than my Facebook "friends". I just prefer the ability to select people and only tell them what I want to say without creating tons of groups for everything.
This might be locale-specific, but there's a lot of times I'm notified of a party/get-together via a Facebook post saying "I'll be down at $taphouse tonight at 9, feel free to stop by!" That's a very passive invite, and Facebook (or Twitter or G+ et al) is perfect for that.
I get what you are saying. The one good thing about Facebook is that it took a lot of that fluff out of my inbox. But could I not have learnt to manage/gloss over it?
The thing that Facebook does well - is tracking down contacts - to add to your 'address book.' It's not so easy in the world of email. That's because we aren't publishing our address books publicly (probably for good reason.)
"Hey, what's your new address?"
"I emailed it to you, go search through your messages"
vs
"Check my Facebook profile"