We won't circle around it, folks: The Circle is called The Circle for a reason.
Sure, this baby is named for the high-tech company where most of the novel's characters are employed, but is that all there is to it? Of course not. Throughout the novel, the geometric circle symbolizes unity, totality, and—uh-oh—closure, all of the things that the Circle is desperate to achieve through Completion.
Yeah, well, once Completion has been achieved and "the circle has been closed," no one inside it will ever be able to get out. In other words, there won't be any way to opt out of the sleepless surveillance society that the Circle is determined to create.
There are other kinds of limitations to a circle, too. Sure, the Circle is trying to make all information—or data—available all the time, but there's still a difference between data and what you actually feel and experience, which can only be quantified—maybe—if you write it down or talk about it. But what about all that stuff that we can't talk easily about? Things like love, art, relationships, solitude?
Those things are excluded from the circle. That means that there's a limit to the circle, even under the best circumstances. If all we've got is the Circle, we're just going to be circling around like goldfish consuming data, not actual experiences.