I might be wrong, but I don't think the question is "will Facebook be overturned by another social network?" so much as it is "are social networks even profitable?"
I don't claim to know nearly enough about economics to even have an opinion on this subject, I'm just saying that I thought the question the article was asking was more about the economic value of Facebook (and, by extension, social networks, though, this extension may be pushing it), than it was about whether or not some competitor would be able to "dethrone Facebook", as it were.
As long as they hold onto their user base, that would be on the low end of what you could expect from them in terms of revenue per user. They are trending towards better targeted somewhat more invasive ads. Virtual currency hasn't really got going yet and as mentioned above people expect them to eventually offer something similar to adsense which would have a high chance of being successful because I think they hold the edge of Google in targeting data.
Of course pushing for more profit could turn off users. Google+ though is showing how hard it is right now though to make a significant dent as an opposing social network.
As long as Facebook don't do anything stupid I find it hard to see how they will be beaten with a similar model. It is more likely that something else will be built which changes the whole social networking paradigm and makes Facebook less relevant.
