CNN did a news report on some research data gathered by Experian Marketing Services earlier this year on the number of site visitors to Pinterest. The report argued that based on this research, which I believe was probably a trend study based on the fact that they have month to month data on site visitors, Pinterest could be labeled as the third most visited site in the U.S. At the end of the report, the author also asserts that data points to the fact that women in the middle of the U.S. use Pinterest the most.
While this research seems relevant to Internet usage in general, I would hesitate to call it valid. The report says that the data does not take into account the mobile users of Pinterest which I believe could skew the data in a different direction considering how many people do have smartphones now with constant Internet access. If Experian can find a way to continue their trend study while including mobile users, I think that they will have a much more valid case on Pinterest use.