Their ad says "OMG, there is no God!" and features a picture of Julia Sweeney, a former castmember of Saturday Night Live and playwright. It looks like this:
To me this is not a very effective response. I suspect that Sweeney donated the money for the advertisement to FFRF but I think they should have used the opportunity to communicate a more specific message about why godlessness make sense. There are many examples of effective atheist billboards to choose from. (One of my favorites is "God is an imaginary friend.")
Regardless, it's good to see FFRF having the wherewithal to respond to the initial religious provocation.
Hat/tip to Joe.My.God