Override Manual Excerpts

Ok this might not work but I'm trying it. Here's the problem.

Facebook sharing totally jacks up all my pretty little paragraphs and grabs random bits of text constantly.

I figured out how to override this by typing in the desired title in the manual excerpt area. Perfect.

Then I decided that I really don't want full posts on my homepage but would rather have excerpts of about 250 words.

Problem? The ones I have made Facebook share ready are now displaying the manual excerpt instead of the first 250 words. I even tried adding the "more" tag manually (the right way not like that). And it still is pulling from the excerpt.

Any ideas on how to override this or other ways to control what gets shared on facebook?