Heddi Cundle, founder of MyTab.co, a San Francisco company that helps people raise money for trips, spent $5 on Fiverr to buy 200 followers last October, when her site started. 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 briefly made the news in July, when Mitt Romney’s Twitter following jumped by more than 100,000 in one weekend — a much faster rate than usual. A flurry of news reports purported to expose the practice of buying followers. “Romney Twitter account gets upsurge in fake followers, but from where?” read a headline on the NBC News Technolog blog.” (The Romney campaign has denied it bought followers.) Similar claims were lobbed at Newt Gingrich last year; his campaign also denied that he paid for any of his 1.3 million-strong Twitter following.

Having fake followers, it is important to note, does not necessarily mean that they were purchased. Unlike Facebook friends, Twitter does not require users to approve followers. In other words, anyone can follow you on Twitter, whether it’s your mother or a spammer.

Twitter followers are sold in two ways: “Targeted” followers, as they are known in the industry, 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 Twitter accounts that are either inactive or created by spamming computers — often referred to as “bots.”

Buyers and sellers see nothing wrong with it. “Buying followers generated by bots is against Twitter’s terms and frowned upon by the public,” Mr. Mitchell said. “However, it is perfectly legal.”