Massachusetts' state secretary — the state's top securities regulator — has subpoenaed investment firm Morgan Stanley over allegations it may have withheld information about the future revenue of Facebook.

Shares in the world's most popular social network were sold in an initial public offering Friday at $38 each. The price of Facebook stock has since dropped to $31 a share — more than 18 percent.

The Reuters news agency reported Tuesday that analysts with Morgan Stanley — the top underwriter of Facebook stock — revised their revenue outlook downward in the days before Facebook went public. The revision was based on a stock prospectus issued by Facebook.

Reuters reports the analysts allegedly shared that information with large investors, but held it back from small investors who bought Facebook stock at $38, expecting the price to rise.

The Securities and Exchange Commission and Financial Industry Regulatory Authority have called for a review of Facebook's IPO.