I won't speak for Sandy, but Quibb's not (necessarily) built with the startup niche in mind. She knew the most likely people to try a work linksharing tool would be tech/startup workers, so she focused on the path of least resistance. As to how she found them and reached them, it was mining her network for a strong first group of users, and tons of emails, coffees, and follow up.

To me the useful lesson of Quibb's first 1k is that your earliest users may not look like exactly like your target market, but need to be the most likely to adopt and provide useful feedback. Especially in B2C, this is likely some subset of the tech early adopter archetype.