The reason I believe that is pretty simple- the government typically doesn't get things done and its been that way for a long time. That's due to a number of reasons. In Facebook and Google's cases its the powerful lobbying ability and best attorneys combined with elected officials' extreme risk-aversion to actually doing things combined with a pretty poor understanding of the situation by law makers. It's far safer to make grandiose statements and finger-wag than to gain support for, and ultimately pass legislation.
People hoping for regulation should be careful what they wish for, as I'd bet that if something does pass, it is precisely these companies that will help author it which likely won't be great for competition.