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For all practical purposes, .co domains are the same as .com domains; that means they are available to everyone and come on a first-come, first-serve basis. One of the biggest advantages of the .co TLDs, besides the fact that they're similar to .com TLDs, is the fact that they are already widely recognized. More than 20 countries, including the UK (.co.uk) and Japan (.co.jp), already use them for their country-specific domains.
One of the early adopters of the .co domain name is Twitter, which snatched the domain t.co, and is currently using it as a link service.