Facebook experienced its busiest ever day in the UK yesterday (June 14, 2009), following the launch of its new personalized username capability.

Over the weekend Facebook made good on its promise to give its 200 million users an easier way to share their profiles - letting users chose an alias for their Facebook profile on a first-come, first-serve basis.

Hitwise reports that the social network was the second busiest website in the UK (after Google UK) on Sunday, picking up 1 in every 19 UK Internet visits (5.17 percent). Traffic to the site was up 9.5 percent on the previous Sunday (June 7th).

UK Facebook Internet traffic

Facebook accounted for 43.32 percent of all UK Internet visits to a social network yesterday, 2.5 times as many as second placed YouTube.

As we reported last week, vanity URLs are becoming an important part of one's online social life, as it makes it easier for others to find you by just typing your name after the service's URL (ie. http://www.twitter.com/pcadvisor). Twitter and MySpace have been providing personalized usernames for a while now.