Hello!
Here are a few I came up with but I don't know if it's good.
Not knowing the sender, can be a danger. (the danger of messaging unknown people online)
Think before you post for it can be unsafe. ( the contents of your post can be harmful...
Don't be a fool to try to be cool. ( foolish people do foolish things online for fame.)
I hope that this has helped/inspired you ♥