Popularity is not so much an illusion as an enigma.  It exists based solely on the opinions and perceptions of others. In other words, its exists, it's not really an illusion - people can most definitely be popular. However, the reasons behind it's existence are ephemeral and transient.  Intangible at best.

Popularity is not always deserved, in fact, sometimes, the nicest and most loyal people are definitively unpopular.  Those that attain popularity do so in a variety of ways.
School or college is a closed circuit environment.  You're required to go (or else be home or private schooled) and the same kids will continually go to the same schools unless someone moves.  This means there is forced interaction between a group of people that don't necessarily have anything else in common, save geography.

This is in complete contrast to the rest of your life.  Once you graduate, you can choose your college and what you study, your vocation, the city you live in, the places you frequent.  These are not generally options for a minor, who is subject to his/her parents' decisions.Kids become popular because others decide to look up to them in some form or fashion.

It may be because they excel at something, like football. It may be because they are intimidating and mean and those around them have more submissive personalities and crave the protection of the more dominant leader. It could be because someone is exceptionally attractive and receives attention based solely on outward appearance. It could also be because a person is exceptionally nice, helpful towards others and retains a healthy sense of self esteem (never an easy enterprise in a high school environment)... even in the face of those that might belittle them.

All of these reasons and more could play a factor in one's perceived popularity.  The best thing you can do is strive to retain a stolid core of self awareness, and not let anyone else's opinion of you alter your own sense of worth.  But, while retaining a strong aura of dignity is not always a simple feat when surrounded by a group of, presumably, judgmental peers, it can be done.  I think most adults, as they get older (and hopefully a little wiser) and escape the high school scene, learn this in some form or fashion.

It's your archetypal coming of age fulfilment.  As you get older you gain a more concrete knowledge of the kind of person you want to grow into.
Be true to who you are.  If you aren't sure who you are, just stop and give it some serious thought – think of your standards and morals and likes and dislikes – this will let you know who you want to be.

Seek out like minded people to be your friends.  This, far more than procuring a broad popularity, will help you to be happy and content no matter where you are in life. Almost as importantly, the number of friends you have is immaterial.  The quality of the friends you keep and what they mean to you, and you to them, is what ultimately matters.