The attack in their small town happened whilst the asteroid/ship/whatever was most likely still on its way to earth... the time between when we last saw it in the sky to when the "first" creature attacked seemed very short, so were there multiple landings? And like you said, if that were an asteroid, then it would've most likely wiped the aliens out as well as most of earth. Must admit that I missed the mention of Shanghai when watching the film, but definitely sounds like it wasn't just a one-off then.