I don't think Futurama is overrated. Clever plot lines with with a little more substance. Sure Family Guy gets more laughs. But in Family Guy, most of their comedy comes from popular culture that is regurgitated to the audience until it's not funny anymore (search "family guy effect"). It has a broad range for a target audience and which is the reason why it is a nuclear family sitcom
With Futurama, the audience has to think a little more to get "it" if they don't already know "it". The target audience is catered more to it's cult followers. Nerds and such. And it's also a workplace comedy, so some people won't understand what that's about.