Mae is the human equivalent of a wet piece of cardboard and she has more boyfriends in a year than i've had in my entire life

Astoundingly, Mae has three romantic partners throughout the course of The Circle. Mercer, before the book begins, Francis throughout, and Kalden later on. Personally, I find it hard to believe that three people could be interested in the human equivalent of a 2-hour-long infomercial, but that's how it is. Fortunately, Mae's love interests are a lot more interesting than she is. Each of Mae's love interests in The Circle are representative of her possible futures; Mercer represents a future for Mae in which she returns to a 'normal,' disconnected life; Francis represents a future of disappointment and full immersion at the Circle, and Kalden represents a future of which Mae has little knowledge, but that may be the best option for her.

Mercer represents everything that Mae is leaving behind at the Circle. He is the one that introduced her to kayaking and the bay, the one that helped her to conclude that kayaking "was criminally underappreciated sport, and the bay a body of water woefully underused." He was such a big part of her past, a part of her development of her appreciation for nature and for things better left unknown, that it is sensible to conclude that he would represent a possible future for Mae in which she returns to her family and leaves the omnipresent Circle behind her. 

Of course, we know that Mae won't choose this future because of her hatred for Mercer and his "professorial smugness" and "antiquarian bullshit." As objective readers, we know that he is reasonable; his apprehensive feelings towards the Circle are entirely justifiable. But Mae, in her cloud of ignorance, ignores his warnings and shuts out the possible future that he offers. His death parallels Mae's decision to willfully deny the danger of the Circle; both make an intentional choice to remove the possibility of a certain future. Mae rejects a future with a return to normalcy, and Mercer rejects any future at all.

On the other hand, Francis symbolizes Mae's future at the Circle; he is boring, pitiful, and definitely not the best choice for her. His awkwardness is a nod to the effect of total immersion in social media culture; when someone decides to commit all of their interactions to an online platform, his or her ability to communicate with others in person deteriorates. 

One of Francis' only laudable actions throughout the book is the creation of ChildTrack. Unfortunately, it seems, as ChildTrack becomes a reality, Francis begins to care more about what the fame from its invention can do for him than the virtue of the invention itself, essentially destroying any envy we would have had for him. This parallels the idea that the Circle's inventions are good, but there is always an ulterior motive that lessens the virtue of the invention. It's also important to note that Francis is beyond terrible in bed. Beyond. The sex scenes between Mae and Francis show how disappointing a future with Francis, and a future with The Circle, would be for Mae.

Even though Mae doesn't get any good sex with Francis, she definitely has a good time with Kalden. Her interactions with Kalden are symbolic of the power of mystery, the power of not knowing. The future that Kalden offers is mysterious and full of unknowns, and Mae doesn't like those. Kalden has a gravity about him; it is this attraction that indicates that he represents the best future for Mae. He is obviously intelligent, suggesting that choosing his future would be the smartest choice for Mae. 

CONCLUSION WITHOUT SPOILERS:
Although it will ultimately be Mae's choice, we can deduce from all of this information which choice is the right choice, both for Mae and for humankind.  

CONCLUSION WITH SPOILERS:
Of course, we all know Mae, and, no duh, she makes the wrong choice. Actually, not even the wrong choice; there are three choices, two of which are good or somewhat good, and this idiot goes and picks the only bad choice. Really, Mae? This is why I hate her, and you should, too. The end.