Facebook Inc.’s struggle continues with no end in sight, especially after its stock prices hit a record low and marketers expressed skepticism whether Facebook ads were really working.

Shares of the world’s largest social network operator plummeted 5.4 percent to close at $18.06 in New York on Friday, the lowest amount since it went public on May 17 with $38 per share.

The fall of the company’s stock to half of its initial value is a shameful outcome, while its earlier projection of stock prices went well above $60 before the much-hyped second-largest initial public offering in U.S. history.

In terms of market capitalization, Facebook lost more than half of its value, falling to $43 billion from $100 billion.

Facebook founder and CEO Mark Zuckerberg, who joined the U.S. biz tycoon club with the IPO, suffered a huge loss, with the value of his shares shrinking from $19.1 billion to $9 billion.

Facebook’s 2012 sales growth outlook turned bleak, too.

Market researcher EMarketer revised down the company’s revenue outlook this year to $5.04 billion on Friday from its earlier February projection of $6.1 billion.

The gloomier outlook is attributed to worse-than-expected effectiveness of Facebook advertisements.

The company’s second-quarter earnings report released in July showed that Facebook’s ad revenue weakened to a 28 percent gain year-on-year, compared to 36 percent in the first quarter and 48 percent in the last quarter of 2011. A big slide in ad growth slammed the company’s second-quarter revenue, resulting in only 32 percent increase year-on-year, compared to 45 percent in the previous quarter.

Some critics question the leadership of Zuckerberg. The 28-year-old is a computer whiz but a novice corporate manager, and they argue that perhaps he should step down and let a seasoned manager take care of the multibillion-dollar business.

However, Facebook is doing relatively well in South Korea.

The total number of Facebook users here has surpassed 8.8 million in mid-August out of Facebook’s 955 million users in total, putting the country on the 26th in the global ranking, according to statistics by Socialbakers.

By Kim Yoon-mi (yoonmi@heraldcorp.com)