On twitter.com, the website, what is the breakdown of browser usage by version number?

hmmm…You should probably ask them directly. Their webstats should tell them.

this is pretty good information, but concerns the internet as a whole, not just Twitter.

http://www.w3schools.com/browsers/browsers_stats.asp

Why do you want to know? (if I may pry)

I’m working on a twitter webapp and want to use html5. So I need to evaluate my fallback strategy for older browsers, specifically IE! Having the stats will help me weigh the options and prioritize.

yeah… that’s a tough one. I would definitely contact the site directly and ask them. They might be happy to provide you with that information. If not, I would just use the stats I showed you and assume it is representative of Twitter users. :/