Facebook's rocky relationship with investors hit a new low Tuesday as shares tumbled another 6% to $21.71 a share. Three trading sessions after the social-networking giant reported earnings for the first time as a public company, concerns about growth and share valuation are far from over.

During Tuesday's trading session, Facebook shares touched $21.61 — setting a new low-water mark less than two months after the company's ballyhooed initial public offering. The shares are now 43% below the $38 a share IPO price when it started trading on May 18.

While the company's results last week roughly matched Wall Street expectations, investors had hoped to see more progress in terms of user growth and in how the company is capitalizing on the rapid rise of mobile usage. Investors wanted Facebook (FB) to "show confidence they could grow the company, and they didn't," says Francis Gaskins of IPODesktop.com