Talking of the security in here on steemit, I guess this as you have it below has addressed that only that you did not talk about the memo key.

1. Log in everywhere by default with the private posting key because this key is the lowest security. It only allows for voting, posting, commenting, etc. It has no rights to send Steem, power up or down, use the market, or even vote for witnesses.
2. Use the active key to make transfers, vote for witnesses at https://steemit.com/~witnesses and handle anything else the posting key cannot such as running a witness.
3. Only use the master password to change the keys as needed. Do not use the master password to sign into steemit.com or anywhere else because anyone grabbing our master password can lock us out of our account. Avoid using the master password to sign in anywhere including third party apps.

But to talk of that much people sharing their private key, that's majorly wrong. That much persons can't be as daft as that and I believe potential scammers ain't as weak as that not to have cleared the accounts either minutes after you dropped those spam like messages in our comments sections or very long before you even dropped the message.
Majority of us only have memo keys shared in those places you claimed to have seen private keys and I remember asking you with a reply to your message but didn't get a response.
What I know about keys on here is shared in this screenshot below (gotten from the FAQ section) and I don't see memo key equaling private key.