The Italian researchers who exposed the growing market for fake Twitter followers are back with new research identifying celebrities and prominent brands with suspicious follower growth.

As reported by The New York Times, security researchers Andrea Stroppa and Carlo De Micheli have questioned the authenticity of Twitter followers for both people and companies. The accounts for Sean "Diddy" Combs, 50 Cent, Pepsi, Mercedes-Benz, and the prime minister of Russia all show questionable activity, the researchers said.

Stroppa and De Micheli identified prominent accounts with massive spikes in follows or unfollows. While their evidence is circumstantial, it suggests that some prominent people and brands could have artificially inflated followings.

Among the researchers' findings: Diddy, who tweets to 8.2 million followers from @iamdiddy, grew his follower count by 185,399 on one day last June. That's 3,063 percent more than his average daily gain. (Last month he lost 393,665 followers in a single day with no obvious explanation; Combs' representative didn't return the Times' call.)

Pepsi copped to unusual growth in its following — it added 71,686 followers on a single day in November 2011 — but its explanation, that it had acquired those followers through promoted tweets, failed to convince the researchers.

The researchers' findings come weeks after they estimated that Twitter, which has 200 million active users, has as many as 20 million fake accounts. The business for selling fake followers could be worth from $40 million to $360 million, they said.

A Twitter spokesman told the Times that "spam is a problem that faces the entire web," and noted several different ways it tries to keep fake accounts off the system. Given the willingness of big spenders to pay real money for them, though, they won't be disappearing any time soon.