Symbolism, Imagery, Allegory
This one's a no-brainer, folks.
In The Circle, the geometrical circle is a symbol of unity, closure, continuity, totality, infinity, and pretty much any other related term you care to imagine. Since the Circle itself—the company, that is—is fast on its way to becoming an all-encompassing, totalitarian superpower, the circle also represents enclosure. Once the circle is complete, those on the inside will never be able to get out.
As Eamon Bailey tells Mae Holland, "A circle is the strongest shape in the universe. Nothing can beat it, nothing can improve upon it, nothing can be more perfect. And that's what we want to be: perfect" (1.44.116).
Mega writer Margaret Atwood has given us a hand by pointing out some of the other symbolic and literary resonances that the geometric circle strikes throughout Dave Eggers' novel. As she writes in her review of The Circle for The New York Review of Books:
The circle motif may be Eggers's wink at Google's "Circles," a way of arranging your contacts on its counterpart to Facebook: but it's much more than that. The circle is an ancient symbol that's had a variety of incarnations. There are divine circles—the Egyptian sun, [and] the vision of the poet Henry Vaughan, who "saw Eternity the other night, / Like a great ring of pure and endless light" […]. (source)
Atwood also goes on to remind us that circles can be "demonic," too. "Dante's Inferno has nine circles," she says, and, for those of you who haven't dipped into Inferno in a while, those circles are the circles of Hell (source).
There are other kinds of limitations to the circle, besides the obvious fact that once you're in it, you can't get out of it. 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, things like love, art, relationships, solitude. What about things like Mae's experience kayaking? Sure, maybe she can talk about it and share pictures of it, but the actual experience is bigger than that. It can't actually be quantified as data.
Experiences like that are excluded from the circle because the circle only deals in data. That means that there's a limit to the circle, even under the best circumstances. It's all about data, not actual lived experience.
So, yeah: Eggers is drawing more than a few different meanings together by lacing The Circle with so much circle symbolism. What else would you expect from a novel that takes a simple geometric form for its title?