When the high P/Es are pointed out about Facebook, the defenders nearly always point out that the platform is immature and/or they haven't monetized yet (this last one is a huge one for Twitter defenders too).
To this I say: you're missing the point and you're basically wrong.
Ads on Facebook are basically spam. They are not things people want to see, not related to anything they're doing, they are at best crude attempts to convert likes into an audience. But they are as welcome as ads on TV are.
Not that advertising isn't (and can't be) effective but there is a time and a place.
On Ebay, if you're looking on Ebay you want to buy or sell something. The intent of an Ebay user is about by definition: you almost certainly want to transact something.
On Google of course you have the intent of search (disclaimer: I work for Google), which is huge.
But on Facebook? Most people just want to share photos, send messages and send updates. Ads are an intrusion.
Probably the most important sentence in this piece is:
> It was not clear what portion of Facebook's $1.6 billion in revenue in the first half of the year came from advertising sales.
Bingo.
Within probably 6 months Facebook will have filed an S1 for IPO. At that point we'll get a far better picture of the sources of their revenue. If the $1.6 billion 2011H1 figure is true I suspect a huge portion of that comes from virtual goods. That's bad.
While I believe that Facebook is a very valuable property I also believe that it comes with huge risks. A few years ago Facebook was scared at the (then) meteoric rise of Twitter. Social networks seem to be incredibly fickle, possibly even generational (as Myspace was). Even with 700+ million users there is always the risk of users abandoning Facebook for the next thing (IMHO).
Also, the games thing is a risk too. Facebook faces the risk of games going elsewhere (eventually as they have an exclusive on many Zynga properties but Zynga doesn't want to be dependent on Facebook long term) or simply new platforms and paradigms emerging, particularly mobile games.
What will be interesting to see is whether Facebook enters the search space in coming years.