Buying listed shares isn't about fast growth for the majority of investors (which are institutional investors such as pensions and insurance).
Investing into listed shares is about the portfolio, not individual shareholdings. The objective for the portfolio is the specified combination of risk & return. People tend to say it's about maximising return for a given level of risk, but that's not really true because what they tend to actually be looking for is, say, coming out 10 years later with something approximating an 8% annual return - which is a lot more like minimising risk for a given level of return.
I understand that. What I'm saying is that Facebook is in a precarious position right now. They don't have a lot of growth potential, but they have a lot of risk e.g. if Google+ takes off, or Apple starts a social network, or some startup comes along and eats their lunch. They're in a market where fortunes change overnight. And with that level of risk, you would expect to at least have a high level of growth...but they're already big. So you end up with a stock which is high risk without high growth.
Of course, if you believe the efficient market hypothesis then it doesn't matter, because the market will value the shares consistent with their risk adjusted return. But the efficient market hypothesis only works if people disregard it... and more to the point, the amount of hype behind the Facebook IPO is epic, making it more likely that the company will be overvalued.
FB has some attraction even if it's just to hedge against your big Google holding.
Sort of... the trouble is that Google is fairly diversified (search, maps, docs/drive, gmail, YouTube, Android, Google+, etc.), so anything that seriously hurts them is likely to be a general industry-wide phenomenon that would hurt Facebook just as much if not more.
All that said, I'm not saying that Facebook is guaranteed to die off in short order. If they end up worth a few times as much in five years as they are now, I wouldn't be particularly surprised -- but if they're worth less than Myspace in five years, I wouldn't be particularly surprised either. The point is that there is a lot of risk, and not necessarily a lot of upside -- if they grow significantly it will have to be by entering new markets, and they haven't proven that they can execute on that.