Rather than assuming the why's, it may be for the best to either ask them directly IRL why or just let it go. I had 30 Facebook friends, now only 14. I find that letting it go, weeds out true friends. Some users do deactivate their Facebook which I find is not worth to worry about.
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"When you begin to realize your own existence and break out of the social norm, then others know you have completely lost your mind." -PerfectlyDarkTails
AS 168/200, NT: 20/ 200, AQ=45 EQ=15, SQ=78, IQ=135