In a statement, USM said it had sold 57.3% of VK's voting shares for an undisclosed sum.
Following the news Moscow-listed depository receipts in VK jumped 6.6% on the day, outperforming the benchmark MOEX index.
VK, which was known as Mail.Ru until its rebranding in October, did not respond to a request for comment.
Its services include the Odnoklassniki and Vkontakte social networks - the latter often touted as Russia's answer to Facebook - and other online brands including grocery delivery services Delivery Club and Samokat.
USM said it wanted to focus more on other companies in its portfolio, including in sectors such as telecoms and metals and mining.
Shareholders in Sogaz, which was founded by gas giant Gazprom, include banker Yuri Kovalchuk, whom Putin has publicly called a personal friend.
As of Oct. 27, Kovalchuk and his wife controlled around half of the shares in the Aquila firm which holds 32.3% of Sogaz.
"The transaction is carried out at market terms," Alexei Miller, who is board chairman at Sogaz and Gazprom CEO, said in a statement.
(Reporting by Andrey Ostroukh, Oksana Kobzeva; additional reporting by Alexander Marrow; Writing by Tom Balmforth; Editing by Jan Harvey and Kirsten Donovan)