Inconsistency is a death knell to any customer experience.
Folks like you and me have expectations... for just about everything. How will our favorite restaurant be tonite? How much traffic will there be on the roads? How will our car drive? And how much uptime to expect from our online services? Everyone has expectations. Those expectations get set from our total experience with a service or product. And when expectations are not met, over time they fall below an internal threshold (which only you know what that is) we seek alternatives, in everything in life, everything. Twitter's downtime issues are no different, its users/customers have expectations around ease of use, portability, extensibility and availability. Each of these affect the customer experience to more or lesser degrees but availability is a biggy! Twitter is an incredibly sticky tool not only because of its ease of use, portatbility and extensibility but because of the human networks that develop on Twitter - because of the relationships that are there. But not even these great elements of a customer experience can withstand having our expectation threshold blown. Once doubt is sown, once expectations are missed, trust is blown.
Once trust is blown it doesn't matter if Twitter fixes its availability issues in a week or a year. Its already lost the opportunity it currently has. One might say, its already lost that opportunity for good. Time will tell.