Sometimes, Social networking sites like facebook, twitter or google plus asked you to re-think your password again and again. This re-think make you feel irritated because you typed the correct password every time. But do you ever think why they do all this procedure because they want to save your account.

According to Facebook's Chief Security Officer Alex Stamos, “the company buys passwords that hackers are selling in the black market and cross-references them with encrypted passwords used on their platform,” tech website CNET reported on Thursday.

These purchases of the password kept the account of users safe from the breaches. "Keeping Facebook safe and keeping it secure are two different things," Stamos was quoted as saying at the "Web Summit" in Lisbon. "Security is about building walls to keep out threats and shore up defenses," but according to Stamos, safety is bigger than that. "It turns out that we can build perfectly secure software and yet people can still get hurt," he added.

According to CNET, when passwords are stolen en masse and traded on the black market, it becomes apparent just how many of them the same are - "123456" and its consecutive numerical brethren are the main culprits. If anyone is using one of these passwords, this automatically makes his/her account vulnerable. Facebook provides a whole bunch of tools for users to make the security on an account nice and tight.

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