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Zynga leveraging facebook messaging for viral growth before facebook banned it

You mean they got people somehow to compose messages to friends, or the invite dialog?

- Instagram piggybacking off of Twitter & Facebook to grow their own network

- Imgur piggybacking off of Reddit & then grew into their own platform

- Red Gifs piggybacking off of Reddit NSFW sites to post short porn clips (idk if it'll be successful to monetize though)

Paypal piggybacking on eBay for growth comes to mind though I'm not sure if it falls in that category.