In this increasingly digital age, there seems to be confusion (perhaps simply among folks of my acquaintance) about information dissemination, but in an attempt to clarify this, here are the rules according to me.*
For news regarding engagements, births, deaths, etc:
1. People who birthed you or who you birthed should be in the first round of notifications. Ideally, these folks get in person notifications, though obviously you may currently be separated by geography (by which I mean a state or three or a country, not, you're upstairs and they are downstairs.) In cases of geographic separation, then your normal mode of communication may be substituted.
2. Posts via email or social networking for others in outer circles are fine, but understand that this may negate any in person notifications you were planning to make.
3. Should you discover said news from a third party and choose to pass it on, please consider that the news originators may still be making their in person notifications, and it may not be up to you to blast it via email or social media.
4. Tagging or texting folks a picture of an engagement ring or baby (or casket although I have only seen that once) is one way to approach this, but keep in mind that depending on the settings of the social media, it may appear to the friends of your friends that they are the ones with the news, leading to many awkward conversations.
*My rules should work everywhere and in perpetuity, but you may know weird people who do not (yet) accept my rules or, it seems possible that these rules may become outdated.