Right or wrong, the reason a large site like Facebook stays large as most people dont want to have to go different places to do what amounts to the same thing.
Would you rather go to 10 friends house each week for 30 minutes each, or everyone hang out at one for the afternoon? Most people would not choose all the running around.
Here's my pet theory at the moment: Facebook stays big because it got a 'critical mass' user base very early and is keeping it through lack of inter-operability with other social networks. If that description sounds a bit like the Microsoft's PC operating system monopoly of yesteryear, that's because it is. This phenomenon also goes by another name: 'vendor lock'. People on Facebook don't have much choice other than to stay on Facebook if they want to enjoy the full spectrum options for interacting with their friends electronically because a huge percentage of 'everybody' is on Facebook and you cannot interact with Google+ users from Facebook so they get left out. If you somehow could force Facebook to be fully interoperable with other social networking sites such that a Google+ user could interact without restrictions with a Facebook user, they could post things to each others timeline, a Facebook user could prune a Google+ users Farmville carrot patches, they could prick and poke each other (or what ever else it is that social media drones spend their time doing) you could soon sit back in an armchair and see Facebook deflate as people defect to a social media site with a better UI, specialist communities more interesting to each individual, etc.....