Ask HN: What are the most successful "X for Y" companies?

I've been thinking about elevator pitches lately, which made me think about the classic "X for Y" pitch model, e.g. "It's AirBnB for Backpackers", or "It's the Facebook for Foodies."

Then I thought about the most successful tech companies from the last decade (e.g. Google, Facebook, Zynga, Groupon) and how I would would pitch them using the "X for Y" model. Maybe I'm not creative enough, but I don't think I can; none of the companies that come to mind were really a twist on an already popular company. Maybe "Facebook is the MySpace for college students," but that feels like an under-sell and it's almost certainly not the way they thought of it at the time.

So rather than trying to scrape Crunchbase, I'll just ask here. What's the empirical evidence for the quote at the beginning of this TC article (http://techcrunch.com/2011/06/05/will-airbnb-ever-be-the-airbnb-for-x/)?

""" “If your company is ever used as the subject of the “X for Y” analogy, that’s success. AirBnB used to be the eBay for vacation rental. Now other companies are the Airbnb of things.”

– far33d, Hacker News """

So...what very successful companies (which I'll arbitrarily define as IPO or acquisition above $100mln) could have been effectively pitched (at the time) as "X for Y"?

(Also let's exclude foreign copycats, since I'm in the USA and don't feel like moving to Chile!)