Facebook has integrated inself so well into society that it will be difficult for a competing company to take their place. You pass by every store, and all you see is like this, like that. Hell, even BMW has a special app dedicated just for Facebook.
Similarly, I also believe that Google will not be replaced. At this point, it will be grandfathered into the interwebs.
10-15 years ago, all you saw everywhere was AOL's KEYWORD
Every company had KEYWORD - XXXXX on their commercials and print. AOL was a giant at the time, integrated extremely well into society, and now look at them.
Same with MicroSoft - they dominated their platform, but today they are struggling to keep up.
People will tire of Facebook eventually, when the next best thing makes them look old and tired and they aren't able to stay current.