Time to put this in a separate thread. The name has come up a couple of times before in FB-related discussions. Here's the earliest occurrence I find in logs; it's fairly new.
184.108.40.206 - - [03/Apr/2014:20:16:13 -0700] "GET /hovercraft/images/dictionary.jpg HTTP/1.1" 200 29465 "-" "visionutils/0.2"
It's mixed in with the ordinary first FB visit, generally towards the end. Question:
Does anyone know for sure what this UA is for? My obvious preliminary guess was that it picks up the one specific image the FB user has chosen for linking. But the timing is wrong: it's part of an initial visit, never more than a second later. Humans don't decide on pictures that fast, do they? Some, sure, but not every time. And one time it requested two different image files associated with the same page. Possibly "Oh, whoops, meant to click the one on the left" -- but again, timing. Do FB users simply have quick reaction times?
In years past I could have confirmed the hunch by looking at hotlinks in days and weeks afterward. But either FB has quietly stopped hotlinking, or the people who like my site have no friends :) so there's no way to follow up.