.com domain is registered much more then 1,300,000,000, this number is only google result for .com extension, this is most powerful domain in over the world any web master who starting his site, choosing .com domain, then if site will be popular he will bay another extensions like .net .org or country domains like .de .jp and others. If you starting your online business better to choose dotcom but if not available then you can register dotnet or dotorg domain, or domain for your country like dotde or dotjp.
Google results for some domain extensions :
This number not mens how many domains registered, this is only numbers how many site are indexed in google with this domain extensions, but this showing more popular domain extensions perfectly.