Interesting, I was looking at the thumbnail pic from the perspective of how it appears on the page. If you changed both the title and the thumbnail at the same time it would be tough to determine which one had the greatest impact on views. But I see you are already experimenting and evaluating (I must have forgotten who I was talking to ).
As far as a plan for promotion drop me an email with your budget (monthly spend and duration), that will give me an idea of what I have to work with. I can even potentially do some off site promotion as well.
Yes I agree the website upgrade has had a lot to do with improved engagement from Facebook. Visitors from Facebook also tended t be more engaged with the website that even visitors arrive via organic search. What is even more interesting, and I'll write about this in the article, is that while visitors from other social platforms has also increased, the relative increase from Facebook completely eclipses the other social platforms and is 88% of all social traffic.
Now imagine if my strategy was to post the actual content directly on Facebook? Think of the value that I would have provided Facebook--without remuneration.