What would warnings would you give to someone new to social media?

What would you tell someone who is new to social media about the problems that can come from posting too much on social media? The bad part of posting too much of your life on social media and how it can backfire on you. Actual examples. Not just the scientificly proven draw backs.
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Real situations examples like people will always be judging you or how you shouldn't give all ideas you have because someone will always have bad things to say about them, etc...
What would warnings would you give to someone new to social media?
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