With the exception of Mint, the other examples were already quite successful before improving their domain name. Establishing causality in these cases is tenuous.
Dropbox went on a tear after their domain purchase, but I suppose it's possible they would have anyway (though I know from personal experience it was tricky to tell people about Dropbox and then try and remember the domain).
http://www.google.com/trends?q=dropbox
(Dropbox.com was acquired in October 2009, right where growth turns from linear to exponential.)
(Not arguing with you, just pointing it out).