SAN FRANCISCO — Is Twitter the next Google, the next Pets.com, or something in between? It may have begun answering that question Tuesday, with its long-awaited first step into advertising.

The startup is trying to make money without alienating the millions of people who have gotten used to Tweeting and following friends, celebrities and others without commercial interruptions. Just as it has through most of its four-year existence, Twitter is treading cautiously.

The new ads, called “promoted Tweets,” will pop up only on searches on Twitter’s Web site, and the messages will be limited to a small group of test marketers including Virgin America, Best Buy Co., Sony Pictures and Starbucks Corp. Fewer than 10 percent of Twitter’s users were expected to see the ads Tuesday, but the messages should start appearing on all relevant searches within the next few days.

The move heralds a turning point for Twitter, which has held off on selling ads even as its widening audience turned it into an obvious marketing magnet and investors poured $155 million into the San Francisco company.

The last cash infusion seven months ago valued privately held Twitter at about $1 billion, even though its only significant revenue had come from giving Google Inc. and Microsoft Corp. better access to its service. The technology powerhouses paid Twitter an undisclosed amount for that right.

Twitter’s seemingly ambivalent attitude about making money reminded some Silicon Valley observers of the profitless Internet startups that wooed investors during the dot-com boom of the late 1990s, only to crash and burn at the turn of 21st century.

Twitter co-founder Biz Stone defended the company’s “slow and thoughtful approach to monetization” in a blog announcing promoted Tweets, even as he recalled a joke Comedy Central’s Stephen Colbert made at his expense during an interview last year: “So, I assume that ‘Biz’ in ‘Biz Stone’ does not stand for ‘Business Model.’ ”

The ads should give a better inkling about whether Twitter will be more like Google or Pets.com, whose most valuable asset turned out to be a sock puppet.

Google took several years after its 1998 inception before it began selling short ads next to its search results, spawning one of the world’s biggest marketing vehicles with ad revenue of nearly $23 billion last year.