I just go over to their houses or invite them over to mine. I've kept up with many of my Google friends, and we usually hang out over homemade pizza every 2 weeks. There are others where I go to their parties or other events or just invite them out for dinner every couple months.

Even in the most company-townish scenarios, I can't really see a company forbidding you from leaving the premises. If you wanna stay friends with a Googler, just invite them over.