[MagicClam]

What defines the "size" of a company in this context is the number of users/buyers, not the number of listed companies. Thumbtack may have half as many pieces of company info, but I bet the number of users that go to Thumbtack for info is tiny compared to Yelp.

I once read that the Yahoo search ad platform has more advertisers than Google, due to more lax standards. Does that mean that Yahoo ads are bigger than AdWords?

[AznHisoka]

Good point... I also thought the title was very misleading, and just lame, to be honest. Also Yelp isn't even in the same space.. Yelp is more for local businesses, not for contractors. Thumbtack is more like Angies List.

[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.