From the summary, it seems like she's talking about much more casual interactions than one has with Sandbox Friends. I'm not talking about people that you have passing interactions with, but people that you actively engage with--co-workers, classmates, that kind of thing. The sorts of people with whom one has an ongoing relationship with, even if it's limited to a very narrow sphere (or box). How many times have you worked closely with someone in a job for a year or two, seen them every day, been quite friendly, and then never seen them again once you or they go on to another job?