Monica, Phoebe, Joey, Chandler, Ross, and Jennifer Aniston's character were never affected by the terrorist attacks of 2001. In this perfect utopian New York does terrorism exist, or was "The One Where We All Stop And Think" just a little too much to handle?
My grandmother died in 1998 during the run of "Friends" and the show never once talked about her death. So, whenever I watch the show I watch it with the hope I might see Me-Ma sipping a hot cup of Joe in the back of Central Perk.
On March 24, 1990 the last episode of "Alf" ran on NBC. However "Friends" didn't talk about the shows cancellation anytime in their long stint. Which means in their world the friends were able to watch up to the 18th season of "Alf."
They did once reference the James Cameron film "Titanic" therefore acknowledging the events of of April 14th and 15th, 1912. But no explicit discussion of the 1500 plus death toll took place, so we can only assume to the friends, The Titanic Disaster was simply a work of fiction.