Google is the exception. Or rather, search is the exception. You can make money off of search advertising because people are explicitly looking for things when they perform a web search. So if you show people good ads, they will click on them.

But how many people are going to click on ads when they're trying to connect with their friends on Facebook? Facebook's entire premise with respect to advertising is that they have more information about the user, so they can target ads better. But it doesn't matter how targeted an ad is if the user isn't interested on clicking on it when it's presented.

> Facebook is now making a non-trivial amount of money from advertising as well.

There's a big difference between "non-trivial" and "enough to turn a profit".

I don't see how the value of the users' attention determines whether or not they are on the service. If you put out free food on the street, people will come and eat it, and you will have their attention. Doesn't mean you can turn a profit doing that.

> Google+ and FB are in direct competition for the attention of the same users.

Yes, they are, but I have yet to see "user attention" be properly monetized on a large by anything other than search engine advertising. Facebook is still trying to find solid revenue sources. Their new gifts platform is the latest example of this.