Ok, so i didnt see anything about this in the main sections, but IM is a very very common use of the internet, so i thought id post some info i have on it when used with Mobile Broadband as well as give others the chance to post their experiences.
Before starting, id like to say Content Control seems to block almost all forms of IM, so you may need to remove it (i think theres 2 levels of CC? The first one blocks IM)
First of all, the types of IM i use are MSN and IRC. The most common client for MSN is Windows Live Messenger, and the most common clients for IRC are mIRC, Mibbit.com or freejavachat.com
Now, when using MSN, using Windows Live Messenger is a no-go, it uses up large amounts of memory at a very fast rate and can be very data-heavy. To combat this, i use "Miranda IM" when on Mobile Broadband. This is a small, low memory client which allows you to access all common IM networks and IRC, all from the same program.
Keep in mind all my results are only from MSN and IRC being connected.
When leaving Miranda idle, you can expect to use less than 1 MB an hour, if you only have MSN connected. Even an active conversation will not use too much data. With IRC connected, and connected to an active Channel, your probably going to go up to 4MB an hour although it completely depends on the channel(s) you are connected too.
Since i dont use any other networks, it would be nice to see what other people find with their chosen messengers or how they find IM on Mobile Broadband. I'v found you can keep the data usage at a minimum when using the right programs rather than the very heavy usage common clients
I use Miranda IM but there are others out there with similar qualities - i have used Trillian in the past as well. If anyone else uses a different program, or has something to add, please do - im sure most of us would like to know more about IM while on Mobile Broadband
Hope this helps anyone using IM