Update: missed a really obvious one--Twitter. It launched as twttr.com and was nearly impossible to explain to people. I don't think it would have gone mainstream without the URL that includes vowels.
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).
But both Dropbox and Facebook were building brands around those particular words, yet had domains that were not the branding words (getdropbox.com, thefacebook.com). Facebook also started out initially as a college-only thing, then opened up to the world, so it's hard to say their growth was linked only to domain name changes.
In either case though, (the)facebook.com could have just as easily been "allmypals.com" or something similar where the branding matched the domain name in a more direct fashion.