There’s a new argument against Groupon’s huge, upcoming IPO (which is estimated to be around $20 billion) — and this time, the argument comes from advertisers rather than the businesses the deals site could hurt.

According to Ad Age, marketers are increasingly focused on the top 10% of U.S. households, because they control about half of U.S. consumer spending. But group buying sites, like Groupon, sell to an audience whose income falls below that 10%.  For most marketers, then, those consumers are an unattractive bet.

Besides the fact that Groupon won’t receive these high-end advertising dollars (that probably won’t hurt the behemoth too much), the statement the advertisers are making about Groupon’s audience is pretty meaningful.  Basically, they are reluctant to advertise to Groupon’s subscribers, which is a silent statement that those eyeballs simply aren’t worth it.  There’s a huge income divide in the U.S., and Groupon’s core audience has far less cash to spend. As such, marketers would rather focus their attention elsewhere.

This argument sets the stage for the idea of a social-media-driven dot-com bubble.  It follows on the heels of the argument that, hey, sites like Groupon are only middlemen.  They don’t actually create a product, they’re just a means to get people to buy things made by other businesses — and how could that possibly be worth upward of $20 billion?  Now, if advertisers are claiming that the people who are supposed to be buying aren’t worth their ad dollars….well, it could be the first hint of a grim future for Groupon and its ilk.

So, is it time to ask whether the new wave of dot-com investors should start fretting?  And we don’t mean the early investors, many of whom have already sold their stock, but rather everyone else.