It's likely that the code is not coupled. Fred Brooks studied OS/360, which was essentially one big component. Facebook is, presumably, many separate components that don't depend on each other in any way, and so the "teams" are much smaller than "all of Facebook".
If you watch velocity on individual components, you'll probably see sub-linear scaling as Brooks predicts.
The messaging back-end is probably completely decoupled from the stuff running other components of the site. I would imagine everything interfaces with each other, and as long as you don't break the interface, things could be changed pretty orthogonally.
Indeed. They have quite a toolchain and a wide variety of libraries. While a change in the javascript framework or PHP compiler affects everything, it isn't dependent on everything.
"Facebook is, presumably, many separate components that don't depend on each other in any way"
Based upon the public statements about their build system and their revision control system and pain when trying to put it in Git I would be confident in saying that is false and that the source code is one big intertwined ball and no separate components.
Edit: Also based upon the way they treat public API which is always better than internal API even if they had internal separate components one could only imagine the horrors of api design (or lack of) you might find.
If you watch velocity on individual components, you'll probably see sub-linear scaling as Brooks predicts.