ABC'S "20/20" REPORTS ON "THE ULTIMATE BETRAYAL," FRIDAY, MARCH 29

How well do we really know the people in our lives? The harsh reality is that, as well as you ever know anyone, you can only really know what he or she allows you to see. Our friends are the people we invite into our lives, the people we turn to for advice and support in good times and bad. Sometimes, you think they know you better than you know yourself. But what happens when our friends commit the ultimate betrayal? Tune in as "20/20" takes a closer look at the people in our lives and raises the question... whom can we really trust? With incredible tales of real relationships gone awry, "The Ultimate Betrayal" airs on "20/20" FRIDAY, MARCH 29 (10:00-11:00 p.m., ET) on ABC.

How Well Do You Know Your Boyfriend...: He's the handsome guy sitting in the seat next to you the next time you board an airplane. He's the guy you might want to strike up a conversation with or even exchange numbers with. This might sound like a typical tale of boy meets girl, but what Soraida Hicks says happened next will make you question ever saying "hello" again. David Muir reports.

How Well Do You Know Your Facebook Friends...: You probably have hundreds of them, maybe even thousands, but what do you really know about your Facebook friends? All it takes is just one simple click to make a friend, sharing personal information with someone you have never even met. These online relationships seem innocent, but what would you do if that person arrived at your doorstep? Meet one man's mission to do just that. "20/20" follows one photographer as he attempts to meet all of his Facebook friends -- individually, in person, anywhere they are in the world. Paula Faris has his story and the story of a couple whose sharing of travel plans led to unfriendly behavior by a Facebook friend. Plus, the good, the bad and the ugly of Facebook.

How Well Do You Know Your BFF...: A small town girl leaves Washington for the glitz and glamour of Los Angeles. Like so many other Hollywood hopefuls, Meili Cady hoped to make it big on the silver screen, but the 19-year-old also longed for friendship. One day the aspiring actress was introduced to Lisette Lee, a wealthy young socialite, and the two women formed an instant friendship. As time passed, Meili struggled to make ends meet - until her good friend proposed the perfect solution: take a position as her executive assistant. The job seemed simple enough: transport luggage between L.A. and Ohio via private jet and don't ask any questions. As Elizabeth Vargas reports, Meili says she had no idea that her troubles were just beginning...she had become a drug trafficker.

Ultimate Betrayal...Revenge: Elizabeth Vargas reports on the dark and twisted world of murder for hire.

"20/20" is anchored by Elizabeth Vargas and David Muir. David Sloan is executive producer.