Do you? I have to believe you don't.
The ad dollars generally just flow to whatever is the biggest, whatever is believed to deliver the highest ROI. It doesn't even have to be good, just large and slightly better than everything else.
Tumblr didn't become Facebook or Twitter. But it could have. If something ever manages to deliver ads that are slightly better than FB or Twitter than those disappear as well.
The bar is simultaneously really low and really high. Low because all you have to do is create a platform that delivers ads with an ROI that is slightly better than nothing, at scale. High because that's actually really hard to do. In some cases "better than nothing" is actually too high a bar. A lot of businesses—maybe even the majority—would be better off spending $0 on FB. Google is the only one who can consistently provide a positive ROI, reliably, for most of their customers.
For a while it looked like Tumblr could have done it. They had the users, the growth, the content and probably more data on what people ACTUALLY liked than anyone else has ever had.