Six Apart - Mena's Corner: The Ups & Downs of a Successful Service
I'm only linking this because it reminds me that scaling issues can have non-linear effects. Here's a succesful service, who's use is increasing, if not linearly, at least following some, presumably smooth curve. But then a particular hard limit is reached : the data-centre capacity, which requires a sudden, step-change at the server end, and there are an accumulated stack of problems.
Many growing services, particularly web 2.0 companies with central servers, are going to be prone to this kind of thing. Such disruptions can potentially kill an incumbant and offer an opening for a rival.