an IM network is for sending messages using that particular network. If you have a jabber account, you can send messages to other people logged into their jabber accounts. To send an SMS (to a cell phone number) from an IM network would require a IM to SMS gateway, which exist, but imo are not worth the cost and effort.

A better solution is to get all your friends to get an IM-enabled smartphone (ie running android or maemo). Or get them to log into meebo, if they are using an iphone.