I know that revenue is often quoted as some kind of measure of the success of a business, when in reality it's extremely easy to run up revenues if you are running at a loss. In the most trivial case, you can pay $1.10 for $1 bills, for example.
Ok, so shopify isn't doing that, but it's hardly different, just a more complex situation. They pay $50 for advertising to get $75 in revenue, which they immediately pay $55 for. That's $105 paid out to buy $75 in revenue.
It would certainly be cheaper to just pay $1.10 for each $1 in revenue. I'm sure the folks at shopify don't think of it this way, and I'm sure they genuinely believe that they will someday change the economics of this and become profitable. However, the balance sheet doesn't support that belief, for me at least, and it sure looks like every customer they maintain leads to more money they are losing.
Perhaps they can become profitable with enough economies of scale. Again, I would question how much larger they would have to get to hit the inflection point, which they haven't done, at least in the financials reported, so far losses are scaling up linearly with revenues (which is to say operating margins are a steady negative % against increasing revenues).
Since they have been around for a long time (12 years) and those same 12 years have seen an absolutely huge increase in ecommerce, it seems like they have failed to hit the required scale despite the most favorable conditions you might hope for. That doesn't make me very optimistic.