That doesn't mean those tables didn't have primary or unique keys. According to that post, in 2010, they had about 10 million users. At a conservative 10 fiel

That doesn't mean those tables didn't have primary or unique keys. According to that post, in 2010, they had about 10 million users. At a conservative 10 fields per user, you're looking at 100 million records. I'm a bit skeptical that they table scanned 100 million records anytime they wanted to access a user's piece of data back in 2010.