The practice of adding people as friends just so they will visit your profile has led to a new form of cheating, which has been dubbed the iSub network. The iSub network refers to the unorganized group of YouTubers who subscribe to as many channels as they possibly can (usually favoring channels that have few subscribers) in the hopes that those channels will appreciate it and subscribe (sub) back.
When a user with very few subscribers receives a new one, they typically visit that new subscriber's channel and watch their videos. Most of the time, this person will subscribe back to their new subscriber. The original user never intended on watching or interacting with this other person, but just got a subscriber bump out of the exchange. Take that instance and multiply it by thousands of subscriptions, and being an iSubber can help rocket your channel up the Most Subscribed list.
It used to be easy to tell who was running this scheme because YouTube had no real way of hiding how many people you were subscribed to. It can be argued that subscribing to more than about 500 channels means you will never have the time to watch all the videos posted by those channels. Some argue YouTube should cap your maximum subscriptions at 1,000. However, in the spring of 2008, YouTube did the opposite and actually created a way for users to hide the number of subscriptions and friends they had, effectively allowing the iSub network free rein to continue their practices without anyone being able to call them on it.
Similar to those who subscribe to everyone, secretly hoping those users will subscribe back, we have the Sub4Sub users. Sub4Sub users blatantly tell you they will subscribe to you if you subscribe to them. They leave comments encouraging everyone to subscribe and add them as friends, promising to do the same in return. And many of these users gain hundreds of subscribers with this method.
Despite its perceived success, both of these gimmicks are good only for short-term number boosting and will not build you a solid audience. Most Sub4Subbers don't watch videos, or they use a second account to view videos where they subscribe only to the users they actually want to watch. These gimmicks are designed to boost their numbers and won't benefit your channel. In fact, participating in these deceptive practices and begging for subscribers will only hurt your channel should you be exposed.