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For sure. Security does not exist if servers that store user's data and passwords are controlled by a single entity. Facebook, Twitter, Medium, Reddit are not secure. That is why Steem is created. Its is a blockchain database that is controlled by witnesses which was elected by Steems Power holders. Steem blockchain don't store your passwords. Your password is mathematically associated using public key cryptography with your account name in Steem blockchain. Thus controlling your password you can be sure that no single entity will be able to access your funds or doing actions on your behalf. It's like in Bitcoin. Steemit.com is a site. A service that use blockchain to show you User Interface. Steemit don't lock your ability to use your data. Anybody can develop apps on top of Steem blockcahin. So if Steemit will try to abuse you you will be able to use another application. So that is a reason why personally me are going to left proprietary social networks. Because of security.

May i know, what public key algorithm that steemit use ?

The password id used as private key ? Which one is the public key ?

The same as Bitcoin does. You even can use the same private key for Steem account and Bitcoin wallet. It is based on SHA-256 and ECDSA.