If you are writing blog-sized posts regularly, there will be a natural temptation to use other media to also share the same messages and to collect similar people who share your interests. Depending on your world, these people might be on Twitter, Facebook, Flickr, Pinterest, or heaven knows what other social network is out there in the world today or a year from today. You'll have to make some decisions about how and where to share your content across systems, and how much of a blog post gets picked out to share to the right place.
The tremendous advantage of writing for your weblog first and then adding your link to some (or many) social media streams is that you retain control over your writing. It's hard to find an old Facebook post, no matter how popular it was at the time, but it's relatively easy to go back through most blog archives and track down what you did before. You can rewrite it, update it, edit it, or take it off line - none of which are easy when you are publishing first on some other platform.
That doesn't mean you should ignore any network where there are people who might be friends or fans of your work. The key is to find the attractive piece of your writing and use that as the lead that draws people directly to your site. In Twitter it's a compelling headline; on Facebook, it's probably a good picture. In any case, keep the original copy of your writing on your blog, and circulate the link to it to bring people to read it.
Most of my blogging writing runs from 300 to 500 words long. That's way too long for Twitter, and stretching the attention span of a Facebook reader who's not used to having a wall of text in their feed full of faces and pictures. Depending on the rest of your design, you'll need to think carefully about which part of any given blog post that you share on some other system and which part of it you have to go to the blog to read. Write a lead paragraph which can be taken out and dropped somewhere else to summarize your message, and a lead sentence that sums it all up in a few words.