opkg install pidgin pidgin-data
There are various plugins for pidgin provided by libpurple that can be installed to connect to MSN (libpurple-protocol-msn), Yahoo (libpurple-protocol-yahoo), etc.
opkg list | grep libpurple-protocol
to see a full list.
You can have instant messaging on the go through GPRS.
Pidgin is a multi-protocol Instant Messaging client that allows you to use all of your IM accounts at once.