With Instagram, Facebook Spars With Twitter - WSJ.com

Of course, those numbers were before Facebook Responds to Anger Over Proposed Instagram Changes (NYT)

"Instagram has quickly become a formidable threat to Twitter. In August, Instagram reached 7.3 million daily active users, exceeding Twitter's 6.87 million for the first time, according to comScore.
Since then, the gap has widened. Last month, Instagram had 17% more daily active users than Twitter, according to comScore data. On average users spent 321 minutes on Instagram in November, versus 146 minutes on Twitter, according to comScore."

With Instagram, Facebook Spars With Twitter - WSJ.com