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For facebook, considering how often stuff is broken.. I Think perl or php would've done a fine job.

For youtube, I think the pull is uploading / watching videos online, and it just doesn't have anything to do with the language they picked.

For Twitter, I'm not sure how it's successful in the first place, but I do feel like there was a lot of enthusiasm in the ruby community, simply because they were based on ruby. So if they picked say php, would they have had that early cache of users? I don't think so.