Whenever you talk online with your instant messaging (IM) client of choice, your conversations can be, and in all probability are, recorded, monitored, and read. Any data which travels over a network can be viewed using programs known as packet sniffers, with some specially crafted programs..
There is however a way to protect yourself against this:
Encrypting your conversations.. It's an easy and necessary step to take, and free 3rd party products are available for most IM's.
Here's a list with some of them.
protect me
If you use an IM not natively supporting encryption, and not shown on this list; Google is your friend