I think it influences success, but predicting that influence... is not easy
Keep in mind too, that the backend you launch with will not be the same as the backend you have at your multibillion dollar IPO. It _may_ still be written in the same language, but thats certainly not a given.)
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.