As your friendly neighborhood source of information that is somewhat useful to you and sometimes also serves as a buzzkill fact-checker for things going around on the internet, I feel it is my duty to inform you that this claim about Solo cups is not true.
From the Solo cup PR folks:
“Although the lines on our Solo cups match up pretty closely with common liquid measurements, they aren’t meant for that. This is simply some Solo cup folklore.”
Solo cups are actually square bottomed now anyway, clearly to provide a more stable base for beer pong.
People talk a lot about being “skeptics”. Some of you may not understand exactly what this means. In a sense, it means that all information is subject to supporting evidence, and in the absence of said supporting evidence, its truth must be called into question. Another way of looking at that is any time you see text on a picture online, assume it is not true until you prove otherwise.