Users are teenagers who don't pay for the service. Their only source of income is advertisers. What was a company like GM thinking? Kids don't buy new cars.
Yes, they have many eyeballs at the moment, but very few ways to monetize on them. The only thing going for FB is that they are currently considered "cool." But, the older FB gets, the less "cool" it becomes.
This is a trend that may be suitable for traders, but it's not suitable for an investor.