While individual elements that these four companies use may not be "usable", doesn't usable, in it's most basic terms, mean that people understand it and can use it efficiently? For example, link colors. The default ones are probably not the best, but to use anything else now is confusing. So, following the example of these larger companies is probably going to result in a more usable site, simply because everyone's learning to use the web via these companies.

What I mean is, can't something become usable that previously wasn't, because it becomes standard?

Brian