They can, but they do not. Why send an email to each person, when you can just post baby pictures or what you are doing to everyone on your feed? Then you don't have to worry about missing someone.
You could just as well use an address book group to send email to everyone or to a specific group. But I worry much more about spamming people with stuff they're not interested in than missing someone.
Sharing on Facebook is a passive thing. You put a picture on your profile and your friends decide if it's worth their time to look at further. You never explicitly told them to look at it, you merely gave them the opportunity. It's like hanging a picture on the wall of your home.
Email is a very explicit share. You are telling someone to look at this, because you sent it to them. This is much more personal. There's a social stigma in over-emailing, as much as there is a social stigma in not reading the emails a friend sends you. Emailing pictures is more like calling a friend. Yeah it's nice to hear their voice directly, but what if they don't want to hear what you have to say at this moment?
Email has changed. I don't reply to every link or picture people send me, nor do I expect a reply on each and every email I send.
There are certainly different social assumptions between email and Facebook, but the differences have long started to blur, depending on what context we're talking about (e.g work vs friends, etc.)
I agree with fauigerzigerk. I would say that email can, at this point, be used to "bump" stuff, to inform people, etc. without commitment or expectancy of an answer. At least, that's the way I use it.
