College is the gilded age of friendships. At no other point in your life are you more fully immersed in your peer group. You live together en masse, you share numerous experiences in the forms of classes, exams, campus jobs, and events. Most importantly, you live in close quarters. Certainly your friend group is of a certain size, but you also have a massive acquaintance circle full of people that you know in connection to your coursework. This acquaintance group isn't one that you necessarily hang out with all the time, but you do greet each other in passing. College is full of social connectedness.
One of the most difficult transitions that any young adult makes after graduation is the emotional loneliness and solitude of the adult world. Certainly there are new levels of freedom, but along with that freedom comes the necessity of work in order to pay bills and to support oneself. Further, now that you're no longer living so close to all of your friends, you find that your number of friendships drops precipitously, especially if you've moved to a new town. Months after graduation, there will be a large subset of your college friends with whom your only interaction is seeing stories about them pop up on Facebook. It's important to stop putting time into relationships that are going nowhere. It may be time to edit your friendships.
From time to time, it's ok to cut the line on a friendship. You have a limited amount of time to devote to your social life, and so when you sink time into friendships that are over, it can be detrimental to your current and future relationships. It's ok to move on, appreciating the value that a friendship had for a time, while respecting the needs and opportunities of your new friendships. This can be especially challenging when you felt a strong bond with someone, but it seems that all of the effort to keep a friendship going is coming from your side.
When considering which friendships to edit, be patient and deliberate. Edit first the relationships that have been over for a long time. Give everyone a chance to move through what might be nothing more than an extraordinarily busy time in their lives. Editing friendships shouldn't be a knee-jerk reaction. Rather, it should be a fair and just determination based on the facts. I'm not suggesting a "You're dead to me" approach. In fact, I'd strongly urge you to not sever ties, simply stop making efforts to keep things going. At the same time, be open to a friendship naturally being rekindled at some point in the future.
While admitting that a friendship is over can be difficult, especially if you've shared many personal things with someone, appreciate the purpose that they served. A friend may have counseled you on a career move, helped you through relational struggles, or been with you at a time of grief. Be thankful for the fruit that a relationship bore and for the opportunity to share a part of your life with someone. Understand also what role you might have played in their life and how that helped you grow as a person.
When life has changed and relationships become inactive, divert your precious resources to new and current friendships. People deserve to have your full attention and efforts at forming new bonds, not someone who's stuck in the past. Be ok with editing friendships from time to time, understanding that your renewed efforts will help your friendships of today blossom.