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Your user number on MetaFilter is at once an identifier and a badge. Each member of MetaFilter has a unique user number that depends on when they first registered; user numbers increase consecutively. From a technical standpoint, your user number helps identify you to the MetaFilter system and also forms the basis of your profile on MetaFilter (the URL for your profile is http://www.metafilter.com/user/(your_user_number)). Many potential MeFites initiate the sign-up process but do not complete the $5 registration fee transaction. Those incomplete sign-ups reserve a user number, but no username is ever assigned to the number. As a result, many user numbers lead to non-existent profiles.
From a social standpoint, user numbers serve as badges to indicate how long you've been a member for. A low user number means that you registered way back when MetaFilter was first created in 1999. A high user number indicates that you are a new member. Some people classify MetaFilter posters as having three digit, four digit or five digit user numbers; the former are old and experienced hands whereas the latter are less experienced, or at least aren't versed in "the ways of MetaFilter." On November 18, 2004, Matt opened new user sign-ups, with a new $5 one-time registration fee. Users who registered on or after this day are colloquially known as "$5 noobs," and have user numbers around 17580 and up.
Of course, in practice it does not help to classify a MetaFilter member by their user number. User numbers, high or low, are by no means a guide towards quality of post, and even very new members may have been reading MetaFilter for years and are more familiar with the environment than members with far lower user numbers.