Internet and social media sites Twitter, Square Inc. and Living Social are trying to crack down on investors who sell shares in the still private companies through so-called “secondary exchanges.”

Bloomberg news, citing several sources, said the firms are seeking to “cap” the number of shareholders in their firms.

SecondMarket Inc.and SharesPost Inc. are open online marketplaces where privately held shares of firms such as Facebook have been traded.

“Board members and companies are getting more proactive, putting in procedures to manage the whole process,” Hans Swildens, whose San Francisco-based firm has been buying secondary shares for almost 12 years, told Bloomberg. “They want to keep the shares in the family. They don’t want it to be the Wild West.”

Read the Bloomberg report here.

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