During today's Facebook earnings call, head honcho Mark Zuckerberg told investors that, overall, people spend "more mobile time on Instagram than Twitter," pointing to comScore numbers to back up the claim. Not a bad justification for that huge billion dollar investment.
That factoid shouldn't be as shocking as it sounds initially. Even though both Instagram and Twitter are both just ways of posting little fragments online, Instagram is way more engaging, and is designed as essentially a mobile service. You are much more likely to burn time looking at those pretty pictures than banging out 140 characters and reading other people's tweets. Plus, because Instagram is a mobile-based service entirely, Zuckerberg's numbers don't take into account that a lot of people read Twitter while they're sitting on their desks.