I guess Sugar Labs partipates to the GSoC for the same reasons the
Free Software Foundation does it through the various GNU projects:
it's a way to write and promote free software that comes with no
strings attached.
But the question is: should a free software INSTITUTION use FB?
While it may be difficult to leave Facebook at the individual level,
I'm sure it is not difficult at all to leave Facebook when you're an
institution.
What will FB do: show you picture of the Stallman crying because you
leave FB?