Many historical organizations are working to connect with their communities through social media. See below for some (but not all!) examples:
Using Twitter?
See the AHA's blog post, "Five Ways for Historians to Use Twitter" for more ideas!
Twitter is also an excellent way to learn more about digital history/humanities. In addition to @dancohen and @dhnow, start with:
Using Facebook?
Research guides for disciplines and courses related to history: