The Practice of Asking for a Facebook Password
The practice of asking for a Facebook password is wrong. You should refuse to give it and, in fact, reconsider whether you should join such a company. Facebook has already issued statements in the USA, in court, that the practice is unacceptable and constitutes unfair labor practice. Furthermore, they have stated that giving your Facebook password is an offense under the Facebook user terms (we all signed it when opening a Facebook account) as it allows someone to pretend to be you (masquerade).
It is an offense under most country laws to use a fake name to engage in online activity (as distinct from a screen name/avatar). It can actually lead to criminal liability both for the user and the HR manager concerned under the Information Technology Act of 2000.