SAN FRANCISCO – Zynga Inc., the biggest maker of online social games, forecast second-quarter sales that may fall short of some analysts' estimates as revenue from mobile titles fails to make up for a drop in users playing games on Facebook's website.

Revenue in the current period will be $225 million to $235 million, the San Francisco-based company said in a statement Wednesday. Analysts on average had predicted sales of $260.1 million, according to data compiled by Bloomberg.

The number of monthly users declined 13 percent to 253 million in the first quarter, Zynga said. Chief Executive Mark Pincus is struggling to restart growth as more users shift their attention away from games on desktop websites such as Facebook, Zynga's core business, and toward games played on wireless devices, according to Arvind Bhatia, an analyst at Sterne Agee & Leach Inc. in Dallas.

"There isn't that much going on in mobile," said Bhatia, who has a neutral rating for the stock. "That's discouraging. That should be the growth driver. That was one of the key reasons for the upside last quarter."

The shares declined as much as 16 percent in extended trading. Earlier, they advanced 5.3 percent to $3.35 at the close in New York. The stock has gained 42 percent this year, compared with an 11 percent rise for the Standard & Poor's 500 index.

Zynga also forecast a second-quarter loss, excluding certain items, of 3 cents to 4 cents a share. Analysts had predicted a loss of 1 cent.

Zynga posted first-quarter net income of $4.13 million, which included a $8.77 million tax gain, compared with a loss of $85.4 million a year earlier.

Sales fell 18 percent to $263.6 million in the first quarter.