Global spam fell last month to its lowest level in three years in a sign that spammers may be getting a better rate of return by hitting social media websites instead, according to the latest figures released by Symantec.

About 70.5% of all email was spam, a high figure but one that is much lower than a few years ago, when it was well over 90%. Security software provider Symantec calculated the percentage by analysing eight billion messages it processed a day last month, according to the company's latest MessageLabs Intelligence Report.

Spam volumes dipped in March after Microsoft, law enforcement and other companies joined forces to take down Rustock, a large botnet responsible for sending up to 30 billion spam messages per day.

Kelihos taken down

"Ever since then, nothing really filled the void," said Andy Watson, a senior software engineer at Symantec.

Botnets increasingly have come under law enforcement scrutiny. Other botnets including Coreflood and Kelihos were taken down this year, and two major ones, Waledac and Bredolab, were dismantled in 2010. A large affiliate spamming network called Spamit also shut down last year.

Watson said spammers may be putting more effort into social media sites because there is a better return. Spam links on services such as Twitter and Facebook can spread through users clicking on the links, Watson said.

URL shortening

Social media spammers are continuing to set up their own URL shortening services, often with open source software. By using a URL shortener, a user is less likely to see that the link may lead to a questionable website.

Watson said the current largest spamming botnet is "Grum," which is sending about 25% of the world's spam. Grum has been on the scene for a while now and Symantec ranked it as the second-most active botnet last year, comprising up to 470,000 infected computers.

The US is the biggest source of spam, sending 28% of the total number of messages, followed by India at 9%; Russia, 5.7%; Brazil, 4.3% and China at 4%, Symantec said. The most popular topics for spam messages are hawking pharmaceuticals, watches and jewelry, unsolicited newsletters and adult-related content.

The full report is on Symantec's website.