LinkedIn confirmed that at least some passwords were compromised in a major security breach. Security professionals advise people to change their LinkedIn passwords immediately. The most common password used was “123456,” followed by “12345 and “123 456789” All in all, more than half a million people chose passwords composed of only consecutive numbers. A hacker could hack 1,000 accounts with just one password attempt 123456 every hundred or so attempts would yield a compromised account, security experts say.
Source: https://thehackernews.com/2012/06/linkedin-confirms-millions-of-account.html