Is Steem a security?

The simple answer is: NO.

Steem is not listed in the recent SEC suing Binance and Coinbase case. Unfortunately, Steem's price, like many other tokens, plumbed a lot.

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https://www.investopedia.com/terms/h/howey-test.asp  
 
How Do You Determine If Something Is a Security?

The U.S. Supreme Court uses the Howey Test to determine whether certain transactions qualify as "investment contracts." If transactions qualify as "investment contracts," under the Securities Act of 1933 and the Securities Exchange Act of 1934, those transactions are considered securities.

The Howey Test attempts to determine if there is an "investment of money in a common enterprise with a reasonable expectation of profits to be derived from the efforts of others." If so, the transaction is subject to disclosure and registration requirements under the Securities Act of 1933 and the Securities Exchange Act of 1934. 

1. An investment of money
   [b]NO[/b] Steem has no ICO, no premint coins.

2. In a common enterprise
   yes

3. With the expectation of profit
   yes

4. To be derived from the efforts of others
   yes