Shopping rewards company Prodege has acquired rewards site MyPoints from United Online Inc. for $13 million in cash.

MyPoints will complement El Segundo-based Prodege’s online shopping rewards business, Swagbucks, said Prodege Chief Executive Chuck Davis.

“They have nine million registered members and Swagbucks has 19 million registered members,” he said, adding that MyPoints will expand Prodege’s reach. “Our overlap is minimal, maybe 10 percent (of users are on both platforms).”

MyPoints joins a number of United Online portfolio companies sold off in recent years, including floral ecommerce site FTD Cos. Inc., as well as social networks Classmates.com, and StayFriends.com. The company’s revenue shrank 9 percent to $151 million last year.

United Online’s declining revenue and stagnant stock price has frustrated some of its shareholders. The Woodland Hills firm is fighting a group of activist investors, led by former Activision chairman Howard Marks, who are campaigning to replace the company’s board of directors.

The company’s share price on Tuesday closed down 40 cents, or 3.5 percent, to about $11 on the Nasdaq.

United Online declined to comment on the sale.

Technology reporter Garrett Reim can be reached at [email protected]. Follow him on Twitter @garrettreim for the latest in L.A. tech news.