Here's a good user experience quote from a UserTesting.com article "7 Facebook Usability Mistakes That Drives Users Crazy, Part III" when talking about Facebook search and the new chat feature:
Move fast and break things works well when you are a start-up. But it just seems like when new features are released by a very successful company that lack usability – the general sentiment of the proletariat is, "They don’t care about us."
I believe in getting new product out and quickly iterating though improvements based on user feedback, but that is for new product. For well-received, widely used products it is key not to break an already existing implementation and and confuse your wide user base.