Police in St Petersburg have carried out searches on the home and offices of Pavel Durov, the 28-year-old founder of VKontakte, Russia's most popular social networking site.
They say they are investigating an incident in which a traffic policeman was injured by a driver who ignored an order to stop.
Mr Durov denies involvement.
Russian authorities have recently cracked down on websites which provide a forum for opposition debate.
They have passed laws making it easier to close sites they say are extremist or harmful to minors.
VKontakte is far more popular in Russia than Facebook, with 200 million users. In 2011, it refused to delete an opposition group, despite an order from the security services.
Mr Durov, who rarely speaks in public and prefers to post messages on his site, describes his political views as "libertarian".
Investigators confirmed that searches had taken place at Vkontakte's headquarters on Nevsky Prospekt in central St Petersburg and at Mr Durov's home.
They say they are looking into an incident on 5 April near the company's office, when a white Mercedes failed to stop for a traffic policeman and drove into him, causing bruises and grazes.
Some media reports allege that Mr Durov was behind the wheel. No-one has been charged and he denies any involvement. His spokesman says he does not even own a car.
"We are working... and suddenly 20 silent men in leather jackets appear," Nikolai Durov, co-founder of Vkontakte and Pavel Durov's brother, posted on his page of the website.
"I wonder what they are looking for. Or perhaps they are gathering information and planting bugs," he wrote.
Last year, Pavel Durov outraged some Russians by tweeting that Stalin's victory in World War II enabled him to continue repressing the Soviet people.