Lots of current startups in YC are founded by first-employee of previous YC startups. Most YC founders I talked too knew people there before sending the application.
PG said that some startups joined YC with only the application. I trust him! But that doesn't mean that lots of others didn't networked hard. If you have a) A good application b) A good application + a previous positve experience talking with the team, who would you pick?
You only have so much time. Building a great company is the most effective way to get into YC.