Earlier, Facebook was too much confused about their user’s user id. As, there was millions of users registered over facebook with their particular email id and that user was identified by the Facebook with their email id. As a result, it was quite lengthy for the user to type their full email address on the place of user id and then password.
To solve this problem, Facebook used some tricky ideas and resolved such problem. It announced the profile page like facebook.com/arijit(username) for particular user identity at the end of July this year.
Many users started choosing their profile page with their suitable name. Just like I registered my profile page with “arijit57”. This gave a great help to the Facebook to give its users a particular User Identification and at last today it announced happily to its users to put their user-id in the place of their user-id box to login.
Ya another interesting for you "Facebook Users now can tag their fiends."