Not a cop, but I did all sides of child neglect cases (defense, GAL, prosecution) for a bit. I can't tell you the number of times a parent was on the stand, saying that they don't drink anymore, don't party, don't do drugs. A deputy juvenile officer would be there, going on Facebook, to check their FB page and they'd find posts contradicting their testimony.
People put a lot of stupid shit on Facebook and similar sites.