Chris Dixon with a smart post on when to shift focus:
Ask yourself: if you started over today, would you build the same product? If not, consider significant changes to what you are building. [...] You aren’t throwing away what you’ve learned or the good things you’ve built. You are keeping your strong leg grounded and adjusting your weak leg to move in a new direction.
Almost every company I was involved with adjusted the focus of their business model in a major or minor way.
Update for clarity: You will often not realize it, but look for companies changing pricing structure, redesigning their experience, or stopping single services or features. Think about why this happened, very often you see a closer alignment with the core of the offering or a push into a different tier of customers (private vs. business, local vs. global, etc). These changes also often give you a good insight into what works and what doesn't in a company.