UPDATE 3-CBS posts surprise loss as ad sales plunge

* Q1 loss 8 cents per share vs Wall St view 7-cent profit

* Q1 rev $3.16 bln, below Wall St view $3.27 bln

* Sees stronger second half of the year

* Shares down as much as 13 percent in post-market trade (Adds executive comments)

By Paul Thomasch

NEW YORK, May 7 (Reuters) - CBS Corp (CBS.N) reported a surprise quarterly loss and revenue that was well short of expectations as severely depressed advertising sales killed any chance that its hit TV shows would keep it profitable.

Shares of CBS dropped as much as 13 percent in post-market trade after the results, adding to declines this year that have seen the stock underperform rivals Viacom Inc (VIAb.N), Time Warner Inc (TWX.N) and Walt Disney Co (DIS.N).

Results from those companies have been salvaged by the relative stability of their cable networks, where revenue is generated by both advertising and affiliate fees.

CBS's cable business is limited by comparison, depending largely on pay TV network Showtime.