Social Bookmarking 2.0.3 (just released) contains fixes for this problem.
The issue here is that AddThis, the third-party service that Social Bookmarking relies on, is being a little unreliable in serving a file that the plugin uses. @julian.maurice contributed a fix to make the plugin behave better when AddThis isn’t working, and we also switched over to a different URL from AddThis which seems to be working more reliably (at the moment).
In the long run it probably makes sense to just move away from AddThis entirely, as Social Bookmarking doesn’t really use many of its features.