Some police agencies are now warning that it's risky to send party invitations on social media where anyone can see those invitations. The concern is that such invitations on Twitter, Facebook and Instagram can lead to gate-crashing, underage drinking, people using drugs, fights and civil liability for homeowners if anyone gets injured because of illegal activity. Police say all parties advertised on social media should have a sober and responsible adult monitoring all activities. They also recommend that parents monitor their children's social media activity. Police say that if uninvited guests arrive and begin to cause problems, the hosts should notify them as soon as possible.