LONDON — When a SWAT team appeared at Pavel Durov’s door in St. Petersburg, he started thinking about his future in Russia.

He was home alone, and he peered at them through a monitor.

“They had guns and they looked very serious,” said Mr. Durov, once Russia’s biggest celebrity entrepreneur. “They seemed to want to break the door.”

Not long ago, Mr. Durov, 30, was seen as Russia’s Mark Zuckerberg. He founded a social network, VKontakte, which is more popular in Russia than Facebook, and made a splash by publicly offering Edward Snowden a job.

Then the Kremlin tightened its grip over the Internet and President Vladimir V. Putin’s allies took control of VKontakte. Mr. Durov eventually sold his remaining stake for millions and fled Russia in April, after resisting government pressure to release the data of Ukrainian protest leaders.