I think there are 3 ways to do that:
1) Investing a lot of money in ads and PR (many people won't have the resources)
2) Being the first one in a niche with high demand (e.g. AirBnb)
3) Staying afloat long enough to get enough traction.
I've been bootstrapping a software marketplace for quite some time now - https://www.saashub.com. A side-project initially and going full-time for the last few months. I can share, and believe, that SaaSHub is on the verge to crack the egg :) . It had 10k page-views yesterday. My expectations are that once it gets to 50-100k page-views a day, it would have cracked problem #1. Time will tell.