Well, I'm a great believer in a very broad interpretation of freedom of speech. But surely even the most extreme advocates of freedom of speech would not say that it means you have the right to say absolutely anything that you want about anyone with no fear of consequences. Like if you said on the witness stand in court that you saw Mr Jones commit the murder, and then it later turns out that Mr Jones is completely innocent and you were lying because you have some personal vendetta against him, I don't think you would escape a perjury charge by claiming "freedom of speech", and you'd be hard pressed to find many people who'd say that that is a valid exercise of free speech.
Yes, there's a difference between fiction and non-fiction. If you wrote a newspaper article, which was presented as completely factual, in which you said that Tom Tebow uses heroin ... I don't know much about Mr Tebow, but I'm guessing that he does not use heroin ... that would be a pretty clear case for libel.
If you put a scene in a fiction story in which you depicted him using heroin that would be less clear. There was a case that went to the Supreme Court a few years ago where a pornographic magazine printed a cartoon depicting a well-known religious leader of the time having sex with his mother in an outhouse. The preacher sued for libel ... and lost. The court said that a reasonable person would understand the cartoon to be a joke -- a crude joke, but a joke -- and not a claim to be reporting actual events.
What are you trying to accomplish? If your point is just to add some cultural feel to a story, I'd say: Why take the risk of being sued for libel and having to defend yourself? Remember that whether someone can sue you is a very different question from whether someone can win a suit. Can someone sue you? Yes. People have sued for all sorts of ridiculous things. If it's ridiculous enough sometimes the court will order the plaintiff to pay the defendant's legal bills, but very often not. Are a couple of stray paragraphs in a story for flavor worth the risk of tens of thousands or hundreds of thousands of dollars in legal fees?
If your point is just to add some flavor, I would think it would be counter-productive to put in things that are contrary to the person's character, or at least to what is known about him. I suppose that if you said that Mr Tebow was secretly a drug addict that some people who don't like Mr Tebow or his religion would laugh and say, "Yeah, he probably does do stuff like that. Those people are all a bunch of hypocrites." But someone who likes Mr Tebow or shares his religion would probably not find it amusing, possibly even offensive. And plenty of people would be saying, "What? Huh? That doesn't make sense."
I'd think you'd be more successful at adding cultural flavor by saying things that are morally neutral and clearly plausible. More like, "Tom Tebow scored a touchdown" or "He saw Brittany Spears on a magazine cover."
Of course if your goal is to make slams against people or groups that you don't like, that's a different story. If the reason you're writing this book is because you want to attack Brittany Spears or Tom Tebow or pop singers in general or Christians or Libertarians or environmentalists or whomever, then prompting lawsuits or at least threats of lawsuits would be a sign of success.
By the way, bear in mind that references to celebrities can be dated quickly. Sure, 50 years later people still know who the Beatles and Gilligan's Island were. But how many celebrities from 20 or more years ago can you name? Lots of people can't tell you who was president 20 years ago. I think very few could tell you who was vice-president, and fewer still could tell you who ran for president but lost.