They eventually do share the ideas; just not in the beginning. Some companies announce themselves in the beginning and some don't. Eventually we will hear about them when Demo Day comes around. This is a post listing the recent class: http://blog.ycombinator.com/congratulations-to-the-yc-summer...
This is a compiled list of all the companies that have ever attended YC and their status (though it seems it hasn't been updated yet): http://yclist.com/
They are getting a huge database to mine. You share your idea, progress and other things, but they dont even get time to provide a feedback! Not even focus on this or that type. Also dont forget stupid show us if we were wrong type sentences.
https://www.reddit.com/r/startups/comments/39kswc/vc_rejecti...
https://medium.com/@bchesky/7-rejections-7d894cbaa084
I haven't heard any stories where YC used their database nefariously and gave away ideas to other founders. This is interesting though, I'll keep this in mind.