Instagram is growing at an estimated 130,000 users per week

"We sampled data from that API and loaded it into an RJMetrics Dashboard for analysis. This yielded detailed new insights into Instagram’s growth and user adoption.
Please note that these are estimates extracted from a sample set, so there may be a small margin of error (more details on our methods can be found at the end of the article). I provide a full breakdown below, but here are some highlights:
Instagram is currently adding 130,000 registered users per week
Instagram’s 2.2 million users upload 3.6 million new photos per week (or 6 photos per second)
37.5 percent of registered users have never uploaded a photo
5 percent of users have uploaded over 50 photos
65 percent follow nobody, and only 12 percent of users follow more than 10 people
For users who upload at least one photo, there is a 45 percent chance they will upload a photo the following week
Those same users have a 25 percent chance they will upload a photo 12 weeks later, representing a significantly stronger retention rate than Twitter."
Source:  Techcrunch, 10th March 2011