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I think the big stumbling block is that Facebook's growth is getting population limited. That is remarkable to even think about, but they're running out people left on Earth that don't have a Facebook account. So your opinion on Facebook's (financial) value really comes down to how much money you think they can make per user. Their current system seems to be 'show the user ads based on the users hobbies, interests, l…
Facebook works in "over 70" languages: https://developers.facebook.com/docs/internationalization/ One estimate says there are 6,910 languages in the world: http://www.science20.com/science_20/blog/rarity_koro_brand_n... In other words, you're simply wrong that Facebook is running out of people who don't have a Facebook account. I might buy that Facebook is running out of people that don't speak the languages it suppo…
http://en.wikipedia.org/wiki/List_of_languages_by_number_of_... gives a great overview of this long tail - with 70 languages, Facebook probably cover all but a few hundred million people. Simply by comparing the top 25 languages spoken with the languages Facebook support, you can see that several billion people should be able to access Facebook based on language numbers, but yet they only have 900 million users.
I strongly suspect that "working internet-enabled device available frequently" is a much better way to estimate potential Facebook adoption than language support. Having never run the numbers, though, I neither support nor refute that Facebook is running out of potential account-holders.