The study shows that Wikipedia's ascendancy to the top of a large pool of online reference sites may be coming to an end because the community-created encyclopedia has become less welcoming to new contributors.
The research team has warned that the changes could compromise the encyclopedia's quality in the long term.
"It's easy to say that Wikipedia will always be here. This research shows that is not a given," New Scientist quoted Dr Ed Chi, a senior scientist at the Palo Alto Research Center, as saying.
Eight years after launching, Wikipedia contains nearly 3 million articles submitted by users themselves and edited by others in the Wikipedia community.
However, when the research team analysed a downloaded version of the encyclopedia they found its growth has peaked.
The researchers found that the number of articles added per month flattened out at 60,000 in 2006 and has since declined by around a third.
They also found that the number of edits made every month and the number of active editors both stopped growing the following year, flattening out at around 5.5 million and 750,000 respectively.
The data suggests the Wikipedia community is becoming resistant to new content and new editors. Those passive editors who make just a single change per month see around a quarter of their changes erased or modified by other, more active editors (the rate was just 10 percent in 2003).
The resulting exclusion of more varied contributions shifts the balance of power on Wikipedia to those fewer active editors, and in turn could make Wikipedia more like a fraternity than a community-driven social encyclopedia.
Over time, the research team warns, the quality of Wikipedia could degrade as there would be fewer editors checking for errors and correcting the vandalism that accompanies user-gen websites.
The non-profit Wikimedia Foundation responsible for running Wikipedia has launched an internal review to make sense of the data, but says the numbers could be deceiving.
For instance, the higher number of modifications to passive contributors' edits could be the result of human editors removing promotional text added to articles by spam programs, an increasing problem on Wikipedia.
The research team will present their analysis at the International Symposium on Wikis and Open Collaboration in Orlando, Florida this October.