go to ur accounts settings.. where u can change ur e mail and all
there u'll see something like giving ur profile page a name like "www.facebook.com/XXXXXX"
give ur account the name
then in ur mobile, the XXXXX is the username..
for example...
http://www.facebook.com/25th.ArmouredRegiment is my user profile..
and to get into facebook, i gave my name as 25th.ArmouredRegiment@chat.facebook.com
hope that helps..