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It's perfectly possible and reasonable to delete all of your data. It's just hard. We have to do this with our clients if they shift off our platform and believe me, it's not much fun deleting 20-100Gb datasets from a shared database with over 2000 tables in it on production kit. But we do it, because we are honest. Facebook are dishonest. Simple as.
Hmm, I'm not sure what you mean by "our platform", but if I post a youtube video, it gets popular, but I get ashamed of it and decide to delete it again, the video will have spread far and wide to other video sites again. Some for public web pages and the internet archive, for stackoverflow answers/wikipedia entries and SEO rats, and so on. How is "your platform" going to help me delete my embarrassing drunk student…
The company I work for deals with background checks and screening information on behalf of our clients' clients. I could definitely see us safely removing all of that personal information from our system and not being able to recover it. But at the same time, we're a much much much smaller organization compared to Facebook.