Important Security Question: Website Promoted in FB Steemit Group Requires Steem Account Info. How to Check if a Site is Legit?

This might be a bit silly, so I apologize in advance if that is the case.

I have been following a few groups about Steemit in facebook.
(I am extremely disappointed by what the facebook users are asking and talking about in other social media channels, but this is another topic)

A few minutes ago, I saw that a facebook user was inviting other users to sign in his "beta" website which operates on steemit blockchain. When you try to log in, it requests steem username + private posting key. Other websites I have seen (like dmania, busy.org etc) go through steemconnect for the log in.

This is the post that the user invites other users (Group has more than 12000 members!)

This is the website that requires for our steem acount info in order to sign in

Could you please advise how we can check validity of such websites? How can we protect ourselves and what should we advise others? What should we pay attention to?

Any help, guidance, reference to information would be extremely valuable!

Thank You!

_{Image: http://www.thebluediamondgallery.com/handwriting/s/scam.html (CC Attribution License)
And Screenshots from Facebook & Website}

p.s. Please check out this post regarding steemauto by Hive account@emrebeyler:
https://steemit.com/steemauto/@emrebeyler/steemauto-store-your-passwords-in-raw-format