Anonymous and other "hacktivist" groups rose to new prominence in the cybercrime universe last year, and a new report shows that they made some serious mischief.

Verizon's annual Data Breach Investigations Report, released Thursday, found that hacktivist groups were responsible for 58% of all data stolen last year. The telecom giant compiled data breach information from its customers and from law enforcement agencies in five countries.

The hacktivists' success is partially due to the sharp rise in the number of attacks Anonymous and its peers launched last year. Verizon, which has been tracking hacktivist activity since 2004, said that last year's collection of hacktivist breaches exceeded the total from all previous years combined.

That trend is "probably the biggest and single most important change" in this year's report, said Bryan Sartin, head of Verizon's data breach investigations team.

When online hacktivism first started in the 1990s, most of what the attackers accomplished were website defacements and denial of service attacks -- annoyances more than serious problems.

Read more: CNN