It can be used for spam filtering. Surely the people you follow aren't spammers (if they are, you are really bad at following people). Maybe you trust that those they follow also aren't spammers. And maybe you trust that those that they follow also aren't spammer. Presumably, you trust this only up to a certain amount of degrees of separation. Personally, I think this shouldn't be done through followings at all. Rather, there should be account dedicated to certifying users. Each user may seek verification from multiple account. Each user may trust any amount of verifiers. If a user is verified by at least one verifier you trust, your client shall mark that user as verified.