That bookmark service that did great out of the del.ici.ous fiasco (sorry I honestly can't remember your name!).
trunk.ly? They certainly weren't the only ones but were in the right place at the right time to attract a heap of users and as a consequence did some crazy weeks to bring forward a heap of planned features to keep the newly attracted users.
Even if we stretched the 48 hours to 48 days, I am pretty sure this has never happened.