So a little story about Thumbtack. My girlfriend is a chef and she and her chef friends have all turned to Thumbtack at one time or another to seek out private clients. There's a ton of them here in SF since lots of people have money in the city.

One by one, they all dropped off. I've overheard them talking about their experiences. Private client posts gig, asks for bids. Chef buys credits, uses them to bid. Private client opens email, never responds, drops off the face of the Earth. Chef loses money. My girlfriend probably spent $100 without any return. She has since gone on to other much more profitable sources for leads.

Thumbtack of course reaps the reward, but at some point their user acquisition flywheel is going to have trouble turning.