Blog and Social Networks are very different tools of expression (and, for what it worth, online marketing.)
A blog gives you absolute freedom and exposes your thoughts to potentially a huge quantity of people. People, on the other end, can enjoy the things you do without necessarily disclose their identity, unless they actually want to do.
A Social Network page/profile, instead, implies that the majority of your audience is made by those you already know or, at least, you are acquainted with. Yes, I either know about the existence of “public” pages or the possibility of “following” somebody else, but this doesn’t change the point.
To blog is more like living into the wild, where you can meet other peers, predators or none at all (and in this case ask yourself why are you still blogging if nobody cares.) While “living” in a social network is fairly safer but actually less challenging because of its “Walled Garden” design.
While is obviously possible to use a blog to stay in touch with people and a social network to publish contents aimed at a (personally) unknown audience, it would be more efficient to use the proper tool designed for the specific task.
Unless you are left without options, why should you use a hammer to cut a wire and a screwdriver to hammer in a nail?