"Now serving 1,000,000," says del.icio.us. That user count sounds solid — and Yahoo's social bookmarking service has usage data to back it up — but sketchier companies have wildly inflated their numbers before. User counts, just like page counts, get inflated as companies fight for the PR limelight. Let's take a look at some of the worst offenders.
SixDegrees.com was also a study in dot-com PR hype—they claimed to have 3 million members, when really, a "member" was just an email address. Users of this site sign in all their friends, who sometimes signed up (after being bombarded with spam), never to return to the site again—yet they were counted amongst the regular users. Righteous.
FG also refers to Facebook, the site that "85% of college students" supposedly used — back when the majority of U.S. colleges weren't on Facebook at all.
The takeaway: Don't spread user-number hype without checking the math.