Facebook told Voxer it was violating a section of its terms of service related to "competing" social networks, said Voxer Chief Executive Tom Katis.

WSJ: Tensions Rise Between Facebook, Developers

"They clearly had us in their cross hairs, " said Mr. Katis.

WSJ: Tensions Rise Between Facebook, Developers

But before the wind picks up, they attend seminars on building a business, with speakers like Trip Adler, CEO of Scribd, Eric Setton, CTO of Tango, and Tom Katis, CEO of group messaging app, Voxer.

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