http://avvo.com/legal-answers/can-i-trademark-a-similar-name... , just to yank something out of Google very quickly.
The law is not unfamiliar with your objection. In point of fact it has dealt with this question rather frequently. And no, your made up pathological case has no ground in the law, and no, I do not think the law would slice and dice Facebook and Lamebook into separate industries. Separate industries are, as the link says, things that can not possibly be confused for each other, like a tax service and a farming implement company, not "a web-based social network" and "a web site for satirizing social networks".
Taking your point to its logical conclusion, two companies always differ on some irrelevant dimension; the ability to find some trivial difference will not protect you. It's the usual thing I think we computer programmers tend to forget when arguing about law... you have to convince a judge you're not in the same industry. It's not a computer algorithm that can be gamed with a bit of pathological input and a loudly-yelled "TAKE THAT!", despite how it may sometimes appear.
Though your last paragraph entirely confuses me; you express doubt about the court case going in Facebook's favor, then cite an example that I think is actually sillier than the idea that Facebook might spin off a site or two?