If you permit skywalks to make one large building, technically most of downtown Minneapolis and (I think?) Toronto would be one big building. If you define "a building" as you can walk from one place to another indoors.
I don't think interaction scales as well as some think, I lived in a 13 floor roughly 600 person dorm at university and as I think back I never set foot on perhaps a third of the floors, and spent the vast majority of my time on just two floors. So making it 100 floors tall would primarily mean I'd almost never visit about 90 of the floors rather than almost never visiting about 10 of them.
This also happens horizontally, not just vertically. I live in a 580 square mile county. I spend the vast majority of my time, probably over 99%, in about 4 of those 580 square miles. That's just kind of how life is, lots of sleeping, gathering and consuming food, working, raising/educating kids... I believe my county nears the size of the state of Rhode Island, however I don't think social interaction is much different merely because the lines on a map are closer together.
I have lived rural, suburban, and urban, and my limited observation is ignoring each other is a defense mechanism. So in rural areas we were all practically blood brothers simply by virtue of being neighbors, but in urban apartments I would not even know my neighbors names. Suburban is somewhere in between.
So that's 20,000 people over 60 floors, 3 per apartment. That's 100 apartments on a floor! They must be incredibly compact if those numbers are correct.
The average household size is still 3 (bit lower in public housing) with average living space per person of ~140 square feet. Keep in mind that those 140 square feet are measured based on the outside of the building!