Hundreds of millions of hacked account details from social networks MySpace and Tumblr have been advertised for sale online.
In both cases, the logins appear to have been stolen several years ago but only recently came to light.
The incident comes the same month it emerged that a four-year-old database containing more than 167 million LinkedIn IDs had been traded online.
One expert said it was "intriguing" all had emerged in such a short period.
Security researcher Troy Hunt also said millions of IDs from adult dating site Fling - which had been breached in 2011 - had been offered on a hacking forum at the start of the month.
"There's been some catalyst that has brought these breaches to light and to see them all fit this mould and appear in such a short period of time, I can't help but wonder if they're perhaps related," he blogged.
Read more about this story at: http://www.bbc.com/news/technology-36416855