Great UX may make for a great tech story, but by itself it won't lead to huge traction. Great UX doesn't mean great marketing, and word of mouth. In the end, it comes down to providing something that taps into a need, and marketing. The UX part is what makes your product sticky, so people won't leave but it can't attract newcomers.
There are 2 sites that are hardly looked at in the tech space: CafeMom and SparkPeople. Take a look at them, and you can hardly say they have good UX - it's messy, and cluttered. Yet both have over 5 million uniques per month.