As stated, your primary problem is how to attract visitors to your blog. And after your research, you found that one way of attracting traffic is through social networking sites. This created a child problem of how to have solid engagement on your social networking sites which is a tedious task so you want to focus first on one to build a community. All for the sake of traffic.
First, I agree with @Force Flow and @Wayne Liew, since you still have a newly created blog, focus on it. Market your blog locally to your friends and to people you personally know who will genuinely like the contents you are writing. They will be the first to honestly tell you whether or not the things you feature on your blog are interesting.
What is your blog about? Are you good at making images? If you still want to have social media presence, it is good to be a bit VISUAL because photos/images are shared on these SNSs. I personally think your expertise is where you should focus in the beginning. Like building your network by focusing on your strengths first. So, if you know Facebook better, stick to it. If you know Twitter better, then have some quality tweets. What' important is your time management and focus, and your eagerness to engage with your audience.