Google describes it's bounce rate metric thusly:
Bounce rate is the percentage of single-page visits or visits in which the person left your site from the entrance (landing) page.
My site is essentially a single page, with no outbound links. Why, then, is my bounce rate NOT 100%, in fact, it is roughly 60%!!
Google Analytics considers any session with more than 1 hit sent to its servers as a non-bounce visit; it doesn't distinguish between unique and non-unique pageviews.
Thus, if someone views your 1-page website twice (ie, refreshes the page, submits a form, etc); anything that triggers any pageviews or google analytics events, it will consider the visit a non-bounce visit.
That indicates that roughly 40% of your visitors are interacting with the site beyond a single pageview.
(If you were doing some sort of external link tracking, then people clicking the Twitter and Facebook links would send a second hit and would be treated as non-bouncers; however, this does not appear to be the case, so that is not the cause of the lower-than-expected bounce rate).