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No reason both can't be true. There's also the mediating factor that being in a grocery store next to someone doesn't allow for. An unfollow is a lot different than confronting someone face-to-face.
If you meet a perfectly random person in the store, you're effectively anonymous to them because they don't know anything about you. On fb, your friends and family get notified about your behavior towards strangers.
If they pay attention to the firehose of info on their feed. In a grocery store, if you verbally assault someone, there are generally more immediate consequences to your actions, anonymous or not. Same in bars. (I've seen it more in bars than in grocery stores.)
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