I'm firmly of the belief that if there is genuine social interaction to be had, it will be had for the most part. Forcing social interaction (corporate mandated fun) is just another tool to tie employees to an employer to make it more difficult/less desirable for them to leave for better opportunities.
Leaving people to their own devices, you will have a couple of go-getters who hobnob with managers and everyone else cut out of the cliques.
Liking your coworkers is a mutually beneficial relationship for everyone and setting aside structured time to do it makes it more egalitarian.