But either way it will happen. Facebook has been wrestling with the issue of deceased users (a relatively small amount currently), and allows their pages to stay on the site. If these profiles remain, the brilliant comic XKCD asks when will Facebook become an online graveyard.
About 290,000 US Facebook users will die (or have died) in 2013. The worldwide total for 2013 is likely several million. In just seven years, this death rate will double, and in seven more years it will double again. The deciding factor in when the dead will outnumber the living is whether Facebook adds new living users—ideally, young ones—fast enough to outrun this tide of death for a while.
If interest in Facebook peters out, XKCD determines the date will be sometime around 2065. If Facebook continues to be the world's predominant social media site, with people continuing to join, the date will be somewhere in the 2130s. But the inarguable point is that we are all gonna die, and Facebook is our new cemetery.
[All information via XKCD, whose What If series is some of the best stuff on the internet.]