Starting this Saturday morning, June 13, at 12:01am EDT, Facbook users will be asked to choose a username for their profiles. This mad rush to grab a username should cause plenty of headaches if and when the masses decide to swamp the Facebook servers. But Facebook may be up to the challenge and have stated the following: When the landrush starts, users will be able to choose just one username for their profile and for each of the pages that they administer (they will not be available for groups or events). Usernames must be at least five characters long and can only contain letters, numbers, or periods. They will only be available in Roman characters at launch. However, users should be careful to choose the right one: once you have made a choice, you won’t be allowed to change it or transfer it to someone else later on. Facebook isn’t allowing transfers in order to undercut the rampant squatter market that would otherwise develop – although there will still definitely be cases of abuse. Facebook says that generic words like “flowers” or “pizza” will not be available as usernames, though we’re not sure how exactly those rules will...