My (perhaps inaccurate) feeling is that it is most important for Twitter. In my experience most people (except perhaps those both very familiar with the website and pretty techy) are unlikely to try and go directly to a URL for most services based on a name, e.g. typing in www.facebook.com/ycombinator, or whatever the youtube/tumblr/etc equivilents are. Whereas for twitter, if you think you know the name, or even if you just think you have a good guess, lots of people will type it right into the address bar.
Maybe this is because Twitter, right from launch, had a very clear link between "if this is their username, it's also their display name, and also their URL", whereas other services, while they might have done the same thing (often as a post-launch change), it was sold more as a "this makes links look nicer" rather than as a "go straight to an account easily" feature.
This shows the importance of good URL design upfront. Twitter did an amazing job of having well defined REST-ful URLs from the beginning and it was one of the factors responsible for their early success. The more rational, guessable and legible your URL's the easier it is for people to use your web service.
It's _a_ factor. Not _the_ factor solely responsible for their success. It's like having well designed tables and chairs in a restaurant; it makes a difference, but if the food sucks or it's in a bad location the restaurant will still fail.
Maybe this is because Twitter, right from launch, had a very clear link between "if this is their username, it's also their display name, and also their URL", whereas other services, while they might have done the same thing (often as a post-launch change), it was sold more as a "this makes links look nicer" rather than as a "go straight to an account easily" feature.