The source reveals that the video mast is actually an animated gif (or cinemagraph, as these tasteful creations are also known as). The stutter in the beginning kind of gives it away, but still quite impressive they included such a long edit and got it to load relatively fast (coming from reddit where uncompressed gifs run amok).
> or cinemagraph, as these tasteful creations are also known as
Just so you know, this is not a cinemagraph. A cinemagraph is not just a synonym for "high-quality animated gif". A cinemagraph isolates one moving part from an otherwise static picture (and the assumption is that at least some motion is frozen - ie, not everything that could be moving is moving).
Facebook is calling it a cinemegraph (see the HTML snippet in sibling commment) as part of their comprehensive war to redefine every word in the English language to be something of theirs.
Well, on second look it seems like the mast is using <video> for me as well- I originally looked too far and thought the id "fbhome-cinemagraph-canary" led to an animated gif. Apologies on making the wrong call!
Also, as a previous reply pointed out, what they're doing really doesn't count as a cinemagraph- FB needs to stop butchering words.
