Fair use of short clips from movie that I bought I bought movie(s) from ITunes,
and I would like to share short clips from the movies to my friends on Facebook/Line,
and maybe publicly on Twitter.

How do I know whether my short clips usage falls within fair use or not ?
IANAL, but I'd guess that if you have to circumvent any DRM just to copy clips of the movie (which you probably do), then it's probably not OK.  Try taking a screenshot of the movie playing in iTunes (ctrl-shift-3 or ctrl-shift-4 on mac).  If it shows up black on your desktop, or if you get an error message, then you probably don't have rights to share it.  But like I said, I'm just guessing.