Anonymous's February compromise of free hosting provider Freedom Hosting II may have reduced the overall size of the Tor network by an estimated 15 to 20 percent, according to privacy researcher Sarah Jamie Lewis, who works for OnionScan, an open-source dark web scanning tool project.

“We believe that the Freedom Hosting II takedown not only removed many thousands of active sites but also may have affected other hosting providers who were hosting some infrastructure on top of Freedom Hosting II,” Lewis wrote in her OnionScan Report on Monday.

Last month, online vigilantes from Anonymous hacked the FH2 service as punishment for hosting child pornography sites. As a result, visitors to many sites hosted by FH2 resulted in a message that read, “Hello, Freedom Hosting II, you have been hacked.”

Since the attack occurred, disrupting service to more than 10,000 FH2 clients, over 2,000 entities on OnionScan's master list of Tor dark web sites have disappeared, Lewis observed in her report. Moreover, of the 30,000 Tor-based services that Lewis was able to identify from a collective database, 4,400, or 15 percent were still found online by OnionScan. (This dramatic decline in numbers does not mean, however, that that dark web shrank by 85 percent.)

In the report, Lewis also attributed the marked drop-off in Tor sites to the sudden disappearance of encrypted email provider Sigaint.