Well of course they worry about it. I can ditch Google and go to Yahoo, DDG, Bing, hell Lycos might still be around, if the Google homepage takes too long to load.

It's been a while since I saw the story about this, but I came away with the impression that users weren't leaving for another search engine; rather, they were deciding that searching (for whatever often-trivial thing) was not an engaging use of their time.

I'd think that would be of especial concern to something like Twitter.