I had the strangest issue last week, featured images from wordpress would not appear on a facebook post... sometimes.
The story began a few weeks ago while I was in koh-samui. My host of choice had a SAN failure and ended up having 4 days of downtime. Unacceptable under any circumstances, so I've started looking at other hosts. I setup a test site on godaddy for one of my clients, but as GoDaddy was also the DNS server, they moved the site as soon as it was setup leaving me with a choice of fixing the DNS back to the existing host, or moving the site in a hurry to godaddy. As my customer wasn't paying for hosting I decided to most the hosting to GoDaddy. Bad move.
The reliability on GoDaddy was perfect, the speed even wasn't bad (not as fast as my locally hosted box, as you'd expect, but not bad). The issue came because my client was complaining that every facebook share, sometimes the descriptions wouldn't appear in the shared post, and almost always the image was missing.
I went through everything a few times and discovered that if I shrank the featured thumbnail presented to facebook to 100x100, the chance of it appearing increased to about 50%, but was still unacceptable. The links were fine, and my browser was able to see the image_src and pick the image up.
I finally gave up and moved the site back to the old host and immediately it started working correctly. Strange. Lesson learnt - cheap hosting just isn't worth it.