Why does my profile consist of numbers instead of my username?

On Steam, my profile is the following:

http://steamcommunity.com/profiles/76561197964085566/

Notice the numbers at the tail end of the URL. While other users also have numbers for a profile like mine, I've seen many others users with their usernames in their profile instead. It makes it a lot easier to link and remember your profile page based on a short username instead of a couple dozen integers.

Why do some users have a username in their profile instead of numbers and vice versa? Is there a way to change this?

I originally thought it was based on the age of the account since I was a pretty early adopter back on January 24, 2004, but I've seen new and old accounts with numbers and usernames.

Update: I've since updated my profile URL so clicking the above link will renavigate to the following:

http://steamcommunity.com/id/johnjon

Yes, you can change this in your community profile - change the "Custom URL" field to whatever you wish the URL to end with.

Here's the step by step breakdown:

• Load the Steam Community website
• Login, if you aren't already, and make sure you're on the "Home" tab
• On the right side, under "Actions" choose "Edit my Profile"
• Click the "Profile" tab if it is not already highlighted
• Change the value for "Custom URL" - the "http://steamcommunity.com/id/" part cannot be changed.
• Click the bright green "Save" button at the bottom

In your steam community profile, click the Edit my Profile link.

One of the headings on the page will be Custom URL:

Once you enter text in this box and save settings, your profile will then be accessible by this URL as well as your current http://steamcommunity.com/profiles/[id]/ URL.