The reasons for being on a blockchain are many, but perhaps the single biggest is that "ad revenue" only represents a small part of the potential value of an ecosystem. Facebook's profits just topped $1 billion dollars. With revenue of $22 billion, it pales in comparison to the market cap of Facebook which is $332 billion dollars.

So if Steem were to be as successful as Facebook then each year it would allocate rewards worth $33.2 billion dollars, which is 33x the rewards that Facebook could pay its users by distributing 100% of profits and greater than the $22 billion in revenue facebook sees. Early on facebook couldn't have possibly bootstrapped its ecosystem if rewards depended upon actually having profits.

The secondary reason is that a blockchain currency is inherently more valuable than a centralized currency. Blockchain currencies are decentralized and largely independent of governments. So in addition to the "business value" that Steem creates, it also creates Monetary Value.

If Facebook shares could be used as money it would significantly amplify the demand for those shares. Furthermore, if owning Facebook stock gave people power over facebook curation there would be still more demand and value.

Looking at things through the lens of sharing ad revenue is missing 99% of the value of the platform.