Facebook CEO Mark Zuckerberg said earlier in May that the company's top priority in 2012 would be improving its mobile application to create a "transformative" advertising experience, but questions remain about when these efforts will begin to generate revenue.
"Ten percent of overall Internet usage is now mobile, up from 1 percent just three years ago," Greenfield said. "In a mobile world, what is advertising going to look like? We can't answer that question today. I think when we look at it right now, there are some big open questions about monetization."
It's not clear yet whether revenue growth or earnings growth is going to pick up over the course of the next year, Greenfield said, because Facebook is investing for the long-term.
"I don't think they are playing to win over the next three months. They are playing to win long-term and they are investing for the long-term," said Greenfield. "Two or three years out it may be exploding and growing rapidly. I don't know if we have that visibility today, that's why we urge investors from the time of the IPO to be very cautious with this stock until we actually understand the growth rates a bit better."