not knocking all the great stuff YC does, but the analysis here seems to assume that none of these companies/jobs would have been created otherwise, which sounds incredibly far fetched.
It's not far fetched to claim this. I'll admit it's unclear whether Posterous would have launched the way it did, or had any reaction similar to it, if we had not been through the YC program. The homepage wouldn't have looked the same (PG hammered us until it looked good). We may not have quit our jobs. We would not have been as aggressive about customer support. (Thanks Kevin Hale!) That's just n=1, but these stories are not uncommon among YC founders.
I posted my napkin estimate of YC's incremental impact in another comment[0]. It's not perfect, but it's a potential starting point.