From my email response:
Right now I believe the autopost is hardcoded to a certain amount of words. It doesn't take the value from the Open Graph object. Looking at the code, it's the first 20 words. We have an issue in our tracker to use the open graph objects, but I can tell you how to workaround it with a small code change
Go into plugins/opengraph/easyblog/easyblog.php. Around line 265, replace the 20 in the following line (in the getBestText method) with the number of words you want:
return $this->getSelectedText($itemText, SC_INTRO_WORDS, 20);
-Melissa