If you are familiar to HTML, you should be able to implement this easily. You would simply edit the /partial header .hbs file associated with the post.hbs (or similar name). You can find this by editing the post.hbs (or similar name) and looking for something like…
…this is just an example. It should be the first line or thereabouts.
Please keep in mind that themes vary, however, these guys are highly disciplined and they all follow a similar pattern of development.
It could be interesting to have it included into the Publii GUI, but that is up to the coders. I seems like it would be a relatively simple add. Options on the web change quickly. For example, there are a lot of new social media sites and apps available and people are moving away from Facebook and Twitter by the millions every day. I imagine that keeping up with the options available is more than a fulltime job for somebody.