Social networking site LinkedIn and online dating service eHarmony warned that a number of passwords had been breached after security experts came across scrambled files with passwords for millions of online accounts.
The two companies declined to say how many accounts had been breached when they announced the breaches in statements issued yesterday.
The breaches come as the latest in a series of high-profile attacks around the world compromising the personal information of millions.
Last year's attack on intelligence firm Stratfor in December compromised data belonging to former US vice president Dan Quayle and former secretary of state Henry Kissinger.
According to Mary Landesman, senior researcher at messaging security firm Cloudmark, a hacker having access to somebody's LinkedIn credentials along with their eHarmony account might be in a good position to commit extortion.
When somebody had the keys to your business and personal details, that might give them all sorts of powerful information, which they might be able to use for years, she said.