[martian]

When you're building a marketplace, it's important to solve the chicken and egg problem. Search Google and you'll see how Groupon, Airbnb, etc. have solved this problem. Thumbtack's approach was to build its merchant base first, then to leverage that merchant base to attract consumers.

So I think you're incorrect to say the "size" of a company can only be measured in terms of the number of buyers. For younger companies, you have to look at which side of the marketplace they are building first and measure that. For mature companies, I think your critique makes sense: it does make sense to look at the overall size of the marketplace, rather than just one side of it.