Don't expect to show up in the chat room and expect that other students will be there for your students to chat with. People around the world are in different time zones, so you might need to plan carefully if your students are to communicate with students from another part of the globe. The type of discourse that takes place in chat is often disjointed. Students might have a hard time following discussions. Links are sometimes lost with people you are chatting with.
Links can go down, and computers can hang or crash.
That's just the state of technology. Although we at The English Zone make every attempt to have respectable people engage in chat sessions, it is possible that people can enter and type inappropriate things. Although this doesn't happen frequently, you should be nearby to monitor your students' chat sessions. It is an exciting method of communication, and is highly addictive. Try to structure more organized activities, where your students must ask certain questions of other chatters, or perform specific locutionary acts.