I was recently quoted as saying, I don't care if Instagram has more users than Twitter. If you read the article you’ll note there’s a big “if” before my not giving of said thing.
Of course, I am trivializing what Instagram is to many people. It’s a beautifully executed app that enables the creation and enjoyment of art, as well as human connection, which is often a good thing. But my rant had very little to do with it (or with Twitter). My rant was the result of increasing frustration with the one-dimensionality that those who report on, invest in, and build consumer Internet services talk about success.
Numbers are important. Number of users is important. So are lots of other things. Different services create value in different ways. Trust your gut as much (or more) than the numbers. Figure out what matters and build something good.