Tafnut, I have a theory.
While in former times we had radio 'n TV shows and limited news sources in common 'bout which we'd converse around the ol' water cooler (network TV was good fer sumptin', huh?), nowadays everbody's gone their separate ways, listenin' to, watchin', 'n readin' a large variety of stuff that their "neighbors" (whoever they are) ain't never heard of. So, what's our current common ground? Why, e-mail jokes 'n stuff, of course. I'll just bet that if I compared my inbox to yours, there'd be close to a one-to-one match. Pretty much our new common ground.
God help us all!