First, thanks for all the work on your plugin! Super helpful and well made!
I built a custom user contribution page that adds a post from the front end. Only on posts contributed from the front end, when shared via facebook, gave me for the excerpt: "You may use these HTML tags and attributes: <a href="" title=""> ...".
On any other post, it shares and excerpt of the content as normal.
I figured that shareaholic grabs the information from the database, so I looked for differences in the database from a contributed post (that wasn't sharing on facebook properly) and a normal post (that was sharing fine). The contributed post and a regular one both looked almost identical in the database.
The only possible thing I can think of is that the contributed post goes to a "pending" status first, then publish, where as every other post goes straight from draft to publish. Could that possibly be it?
Here's an example of a contributed post: http://www.nathanligo.com/5th-annual-japanese-karate-kyokushin-kan-karate-tournament/
(hit the facebook share button, and it'll quickly be evident.)
Here's an example of a normal post (again hit the facebook share at the bottom to see how it normally works): http://www.nathanligo.com/images-from-shihan-marty-petrovichs-june-2013-belt-test-in-maine-usa/
Thanks for your help!