The cover of this weekend's Barron's is a booming takedown of Facebook titled bluntly: Facebook Is Worth $15 (the stock closed $22.86 on Friday).

In other words, despite the massive plunge the stock has seen so far, it's got a lot more to fall.

The points made in the piece:

The stock is trading at a PE of 48 and a forward PE of 36 times 2013 earnings.

Meanwhile, Apple and Google trading at 16x 2012 earnings.

It's also trading at 10x revenue, twice Google's valuation.

Facebook is struggling with mobile monetization, and the management's response has been "trust us". The fact that it had to shell out $1 billion to buy Instagram is ominous.

Mobile access grew from 9% to 11% in one quarter, a trend that's likely to continue to grow.

The mobile ads it runs that are working are increasingly alienating users... Wal-Mart and Target ads that appear in the newstream are annoying.

Demographics do not favor Facebook. It's getting less "cool."

There are a lot more insider sales to come.

That's basically it: Nothing groundbreaking. But the bear case is simple: A stock doesn't deserve such a big multiple when it's business model is in doubt.