The usual reasons are privacy or disinterest. Not every person is compelled to share all of their personal feelings, activities, private lives with others (even relatives and friends).
Some individuals do not require an outlet beyond their normal social life and peer groups, and many do not have the need for continuous real-time sharing of news or events.
Others are wary of strangers, or are concerned that their posts might be used against them, as informal and unofficial checks on their background and personal history.
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