The biggest vulnerability is that Facebook is a desktop product for monetization purposes. Its mobile products only help in referring users to the desktop product. Basically, what this means is that purchasing Facebook shares is taking a company-specific bet that it will solve the mobile monetization problem. This it admits.
$100 billion is a nice round number. It translates to earning about $5 per user in the near term. Google claim to earn $10 per Android user. You do the math.