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.
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.