In particular, Facebook's user growth rate in the U.S. is slowing sharply. In April, U.S. unique visitors to the website increased to 158 million, up just 5% from a year earlier, according to research firm comScore Inc.

That was Facebook's lowest U.S. user growth rate since comScore began tracking the data in 2008 and was down from 24% growth in April 2011 and 89% in April 2010, comScore said.

At the same time, the growth rate in the amount of time people spend on the social network -- otherwise known as engagement is also decelerating. In April, Facebook users spent more than six hours a month on the site, up 16% from a year earlier. But that compares with a 23% increase in 2011 and 57% in 2010, comScore found.

In some ways, the slowdown is to be expected. Facebook has already grabbed 71% of all 221 million U.S. Internet users, according to comScore. At the same time, people are on Facebook longer than any other site. The more than six hours a month that people spend on the social network exceeds the over four hours a month they spend on all Google sites including YouTube, and the 3 1/2 hours they spend on Yahoo sites, comScore said.

With such a dominant presence and so much time already spent on the site, "declining growth rates are a natural part of the growth cycle,"said comScore analyst Andrew Lipsman.

Still, the falling rates are significant given that Facebook has framed itself as a growth story to Wall Street.

When it went public last month, the Menlo Park, Calif, company commanded a valuation of more than $100 billion, partly because investors expect Facebook to grow at triple-digit and double-digit rates much like Google Inc. did after its 2004 initial public offering of stock. Yet Facebook's shares are off 29% since they priced at $38 ending at $27.10 on the Nasdaq Stock Market at 4 p.m. Friday and the company's market capitalization has sunk to $73.17 billion.

Indeed, comScore's numbers illustrate how little room there is left for Facebook to grow in the U.S., which is the most important and lucrative market for the social network's two revenue-paying constituencies: advertisers and developers.

Approximately 56% of Facebook's 2011 ad revenue of $3.1 billion came from the U.S. alone, according to the company's regulatory filings. "The U.S. is a dominant source of revenue for Facebook," said eMarketer analyst Debra Aho Williamson, who estimates that this year the nation will account for 51% of Facebook's revenue.

Morningstar analyst Rick Summer said comparing Facebook's growth rates to Google's post-IPO might not be an apt analogy given that Google went public at an earlier stage of the Internet's development. "Google had the advantage of increases in ad pricing, increases in the number of Internet users and stealing market share from other Web properties," he said. By contrast, he said, "Facebook is already a dominant Web platform and they've got significant Internet penetration today. Ad pricing is clearly going to be where their growth is going to come from."

Mr. Summer said he expects Facebook to be able to grow into its valuation over the long term. In the short term, however, he said the slowing user and engagement growth rates as well as slower ad-revenue growth are likely to pressure the stock for another year or two.

Facebook still has growth sources. The comScore numbers don't take into account overseas markets and the mobile sector, which are categories where Facebook has room to expand. In its regulatory filings, the company has said it is experiencing substantial growth in countries such as India and Brazil, although ad rates in those countries are lower than in the U.S.

Facebook is still experiencing growth in the use of its site on mobile phones. In March, the company said its monthly mobile usage world-wide rose to 488 million users from 425 million in December.

Facebook acknowledged, however, that its mobile growth hasn't led to an increase in ad revenue because it earns almost no money from mobile ads. Last week, the company introduced new ways for advertisers to buy mobile ads.

It is still unclear whether Facebook's mobile growth is coming at the expense of personal computer usage, said comScore's Mr. Lipsman. ComScore began tracking mobile use of Facebook only several months ago, so there are no clear trends in the data.

Some advertising-agency executives said that Facebook's slowing U.S. user growth rate isn't a concern as long as the company can figure out how to give marketers more opportunities to reach users.

"The story line is less about the users themselves,"said Sarah Hofstetter, president of digital ad agency 360i, a unit of Dentsu Inc. "It's more about how they monetize their user base."

Others say that not all activity on Facebook takes place directly on facebook.com. For example, websites and apps let Facebook users sign in with their Facebook identity, so users might be interacting with Facebook even when they aren't on the website.

"That can understate the real engagement on Facebook," said Brendan Wallace, co-chief executive of San Francisco-based professional networking site Identified, where users can log in with their Facebook user names and passwords.

Still, Mr. Wallace said it is important for developers to take note of Facebook's slowing user growth rate.

Identified, he said, caters primarily to young users, who are a large part of Facebook's audience. But for products that cater to older audiences, Mr. Wallace said Facebook might turn out to be "just one of many platforms that developers consider integrating with."