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how about the same on steemit.com ?

Yes steemit.com is developed by Steemit Inc the company that launched the Steem blockchain and holds the most stake.

We are talking about steemit.inc knows private keys to the great majority of steem accounts.

I heard some of steemians that, they [other wibsites] made some upvotes for there own ID's. Is that possible? can they control the ACCOUNT?

If you gave them keys, why not?

sounds weird. if they can do this, then.........

You should give them weakest key possible.
And change master password (and hence all keys) regularly.

Yes I use the posting key!

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