Following high school, you could be fairly certain when you'd see someone again. Summer was always your best bet because most people would come home and commute to their internships during the day. After college, most are faced with the great unknown. Some may have secured stellar jobs in a new city, some may be looking for jobs.
Gradually, daily texts with the same person become less frequent. In fact, they may stop altogether for a period and correspondence becomes limited to major holidays and birthdays.
That is okay. It's even understandable, really. While I'm not fond of the whole 'I'm so busy I couldn't respond to your text' trope (really? It takes less than a minute to respond), things happen. It's natural to accidentally let the conversation drop, or even be phased out of a friendship entirely, however hurtful it may seem when someone just doesn't respond to your repeated requests to hang out.
After some time, you'll realize that the friends you have left standing from after graduation are a paltry few from your plethora as an undergrad. But they will also be the ones in it for the long haul.
'Things happen for a reason' is romanticized nowadays, but I am a firm believer in the philosophy that one can learn from every and any circumstance in their life, friendships included. Life after academics can be lonely, especially if you didn't end up where you imagined yourself to be. But the friends who are still with you after the arduous journey are the ones whose hearts are in the right place as they supported you during the struggle. Especially as I had a rough time after graduation, I'm lucky in having my eyes wide open in realizing that the people left over are the ones who are in it for the long-term.
The friends you make in college are reflective of who you are then, but the friends you have after college help you become the person you are meant to be.