Unlimited Messaging includes all messages sent to ANYONE on any network except International. So if you have friends that use Verizon or any other network, feel free to text them all you want.
Your Unlimited Messaging Plan also includes IM, provided you use one of the PREINSTALLED IM clients on your phone. However, if you access IM through the MEdia Net page OR a 3rd party, you WILL incur data charges.
Did anyone on your plan download ringtones/graphics/games or access the internet at all??? That could be what your data charges are for.
How could I have used so much data?
Peeps!! 
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*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.