Anybody connected to the network between you and whoever you are talking to can read your messages as easily as if they are standing between you and someone you are talking to. This includes hackers, your i.s.p, the people running the websites or i.m. service, and the government. The main way people work around this is encryption, here is the best way to use encryption with these networks - you will need 2 things:
1. Pidgin instant messenger program. This program can be used instead of many instant messenger programs including A.I.M., Yahoo, MSN, Google chat, and about 15 more. It does not support webcam, but who cares about that. That is what skype is for.
2. OTR Messaging plug-in for Pidgin. This is the most secure method of encryption preventing man in the middle attacks that other encryption methods for instant messaging may leave open.
With this method you login to your msn, aim, yahoo, myspace, facebook, or other instant messenger service using the Pidgin software (after you have also installed the OTR plug-in).
If your friend has also installed Pidgin and the OTR plug-in you can hit the button saying "not private" and then "start private conversation". you will have a question that you type into the box and if your friend types the correct answer he is authenticated as really being your friend and half of the connection is made. Next he asks you a question and when you answer correctly you are authenticated to him and the encryption is complete. The authentication lasts until you remove it, but I would re-do it every other time you chat.