Email is the least secure technology you can pick. For my communications, sharing birthday-pictures and so on, Id like to know that it takes more than to just sniff the somewhere along its path.
Practically speaking, in most cases, whoever is in control of your SMTP server and whoever is in control of the recipients mail server can read the email. (Reading it in between those would usually require deep packet inspection).
Similarly, if you post it on Facebook, people with access to Facebook's databases can see it. The risk seems to be pretty comparable to me, it's just different companies you trust. Certainly sniffing your email off the wire would take a lot more effort than somebody at Facebook reading your posts, and moreover has to be done while the email is in transit.
Email has the option to provide more security through encryption, and is less dependent on a single company.
For communicating anything of importance to my friends, I prefer phone or email, in this way I'm pretty sure that they'll receive it. To Facebook I post chit-chat and interesting tidbits for which I don't care who reads it and who misses it.
As sad and undesirable as it may be, I have a lot more sensitive information than simple birthday pictures in my inbox. Email simply makes sense compared to other options from a usability standpoint, especially when communicating with people who are not technically inclined. It's a risk that many of us still take, even if we know better, because the alternatives are not user friendly or mainstream.
