Right. Let's imagine that Facebook was written in Perl, YouTube in Ruby, and Twitter in PHP, do you think they would still be successful?
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.
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.)