As a comedian, Dan Nainan was blessed with fans. But one thing was missing. "The number of Twitter followers I had in relation to how many people in the world know about me was woefully inadequate," he said. So in June he bought a small city's worth for $424.15, raising his Twitter follower count from about 700 to more than 220,000.

"When people see that you have that many followers, they're like: ‘Oh, my goodness, this guy is popular, I might want to book him,'" said Mr. Nainan, 31.

It may be the worst-kept secret in the Twittersphere. That friend who brags about having 1,000, even 100,000 Twitter followers may not have earned them through hard work and social networking; he may have simply bought them on the black market.

Celebrities, politicians, startups, aspiring rock stars and reality show hopefuls are known to have bought large blocks of Twitter followers.

The practice is surprisingly easy. A Google search for "buy Twitter followers" turns up dozens of Web sites like InterTwitter.com and FanMeNow.com that sell Twitter followers by the thousands (and often Facebook likes and YouTube views). At BuyTwitterFollow.com, users simply enter their Twitter handle and credit card number and see the ranks of their followers swell in three to four days.

"And it's so cheap, too," said Will Mitchell, the founder of Clear Presence Media, a marketing company. Mr. Mitchell said he bought 250,000 followers for $2,500, or a penny each.

Heddi Cundle, founder of a San Francisco company that helps people raise money for trips, spent $5 on one site, Fiverr, to buy 200 followers.

By the next month, "we had about 1,100 to 1,200 people on both Twitter and Facebook, which was amazing," she said. "We needed that to get ourselves going."

Fake Twitter followers made the news in July, when presidential candidate Mitt Romney's Twitter following jumped by more than 100,000 in one weekend. A flurry of news reports purported to expose the practice of buying followers. The Romney campaign has denied it bought followers.

Twitter followers are sold in two ways: "Targeted" followers are harvested using software that seeks out Twitter users with similar interests and follows them, betting that many will return the favor. "Generated" followers are from inactive accounts or created by spamming computers — often referred to as "bots."

The practice has become so widespread that StatusPeople, a social media management company in London, released a Web tool called the Fake Follower Check.

According to the StatusPeople tool, 71 percent of Lady Gaga's nearly 29 million followers are "fake" or "inactive." So are 70 percent of President Obama's nearly 19 million followers.

But Twitter is clamping down. In April, it filed suit in federal court against five spammers, including those who create fake Twitter followers.

That didn't discourage Mr. Nainan. He recently asked about "the theoretical maximum" Twitter followers he can purchase. "They said, ‘You could probably get over a million, a million and a half,' "he said. "And I'm like,‘Why not? I can afford it.'"