Buying in order to sell for profit is speculation. I completely and utterly agree that Steem, as well as many other blockchain techs, is very promising in this regard.

Investing I believe originally means buying into a share of business. Share that implies the right to corresponding share in profits and assets possessed. One can argue that SP kind of does all that. The strange situation here is that Steem as a business has no external source of monetary income. All SP does, from economic point of view, is redistribution of 9.5% inflation of the same SP asset. SP is a share in business that has no profits by design.

There's also lending. Give someone money for a time, expecting them to return more, therefore enjoying profit. That's how SBD is supposed to work, but it currently does not.