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
return $this->getSelectedText($itemText, SC_INTRO_WORDS, 20);
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