Facebook, Twitter, and Google+ all have one very important thing in common: They are social networks. Their goal is completely different than the goal of Stack Exchange, which is to create high-quality content in the form of questions and answers.
While the social networks focus on connecting people, Stack Exchange focuses on building high-quality content that serves future visitors for years to come. The Stack Exchange system determines what's high quality and what's low quality using the voting system, which Stack Exchange tries really hard to make sure always focuses on content, not people.
Anything that turns Stack Exchange into a social network threatens the very existence of the reputation currency system by moving the focus onto the people instead of the material posted. This could lead to certain individuals having higher reputation, simply because of who that person is instead of what that person posted.
Instead, let the social networks handle the sharing. People and systems do much better at what they specialize in.
As an aside, if there are certain people who you'd like to share posts with, check their profile. Many people are on Twitter, Facebook, Google+, or have blogs or ways to contact them. It wouldn't be unacceptable to tweet a question to your followers, where Jon Skeet just happens to be one of them.