1. Out of Sight Out of Mind
Hi, you're face is missing from their day to day, aka your presence is missing from their thoughts for the next four months.
While your study abroad class schedule may make it hard to remember, the academic rigor of your State-side school makes it hard to eat and shower at times, let alone take 20 minutes to FaceTime.
3. They Know, They Saw Your Post
They liked your Instagram, what more do you need?
They simply can't be bothered to download WhatsApp...or for the Cuba travelers, IMO
...doesn't look good on anyone, especially your friends. Alas, it is one of the most undeniable of human emotions, and you can bet that the friends you left back home are doing their best to avoid the the evidence that their own lives are less exciting than yours ATM.
Never mind your friends (for the next few months at least). You're abroad! Live it up now, fill them in later.