I could dig through the code of piston and see if there is any mention of it, or even make the effort to read the whitepaper. But I am just posting off the top of my head here. I believe that the follower/following thing is an additional database controlled by the webserver of steemit.com. I don't think you can find this information on the blockchain. I'd be glad to have someone who knows to correct me, but I have a pretty good idea already that I won't be corrected.
This is something that I have been figuring out for a little while, the last few days. The blockchain only controls the process of interacting with your wallet and posting and voting. Everything else is external. There is absolutely nothing stopping someone from implementing more blockchains that link to this, using apps that talk to both Steem and these sidechains.
To be properly distributed and secure against malicious manipulation, new features would necessarily need a sidechain. I have already had the idea of creating a namecoin-like blockchain for implementing groups within Steem. If I have not missed my guess, there might also need to be an association blockchain for these kinds of social network link databases, like groups, friends, and so on. It would not be hard to add other system interfaces that for example implement peer to peer exchanges, like steem<->bitcoin steem<->dash, and so on.
The sky is the limit, I think. I am no kind of malicious hacker type, I do get a bug up my butt when I see people exploiting weaknesses in a protocol and I am in this position of having such a bad 'reputation' score mainly because I couldn't, instead of disappearing offensive posters (offensive, as in, abusive) who have big power, and I took on a whale on this. Either I am stupid or stupidly brave trying - but the point I am making is that this is just the beginning. We need to foster an ecosystem of steem platform developers. We can add value to this network so easily, and make a mint helping. If this blows up, like I think it will, we could well see 'steem' replacing 'bitcoin' on the lips of the general public who are aware of these things.