When we think of the most popular domain extensions (TLD's), we have to think about them in two different respects.
1.Registrations: volume or total number of domain names that have been registered in a particular TLD because of popularity. Or maybe some other circumstance that has nothing to do with popularity or affinity for a particular extension.
2.Usage: What TLD's are people actually using to build their online business, blog/site or projects with because of popularity, born out of what the extension represents or means to an industry or group of people. An example would be .la, which is the ccTLD for Laos but some have repurposed the domain to stand for L.A. (Los Angeles).