I guess it depends on how you define a successful company. In the capitalist sense, something like http://yclist.com/ is probably most poignant.

I wouldn't say that YC has a huge impact in terms of social media; they definitely help with getting press mentions, but getting engaged subscribers is really up to the founders.

Twitter is not relevant to all companies, but it's obvious that many are leveraging it very effectively. I wonder if investors would be interested in a dashboard like this for all their portfolio companies. The correlation to "real growth" is probably small, but still better than nothing.