For Twitter, you can use the "Tweet" button to send out a tweet about your blog post. That's available right on the blog post's page (turn it on through settings if it's not visible).
Our Facebook page isn't live yet, but we've been experimenting with a 3rd party product called RSS Graffiti to pull our blog posts onto our Facebook wall. It seems to work well so far. You give it the RSS feed of your blog, and it will pull posts into Facebook at your defined time intervals.
Jamie