Like everyone else said, it does NOT qualify as text messages. It uses Data. But obviously if you have Verizon they don't let you take a blackberry home without a blackberry plan which is $30/month. So you're good to send messages day and night.
To use Blackberry Messenger, the other person needs to have a blackberry also. Click on the BB icon. Then click the menu button and "add contact." From there enter their email or PIN. That's send the other person an 'invitation.' Once they accept it's pretty much under your buddies. Then just use it like you were IM'ing them.
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