I've lived in a different country from my parents for many years, also while I was younger. I understand how much you want to keep in touch, be part of his life, hear what's going on.
The thing is, his life is probably very different now and perhaps there's not a big spot for you. Skype isn't "hip" with kids these days, that's for us old people. Email is just ancient, might as well use pigeons. Facebook might be too public, depending on how it's used.
Instead of trying to find a solution yourself, try to get his perspective. Next time you reach him, tell him that you'd like to know how much - and how - he'd like to stay in touch with you. No, you don't want to monitor him, but you want to be happy about the cool things he's got going on, but if you don't know then you can't smile.
What worked for me was to set up a schedule, sort of a recurring date. I knew Thursday evenings at 8 was an hour of NetMeeting with my dad (yeah, it's a while ago...) and we had the deal that sometimes other commitments came in the way but as a general rule we tried to connect then (and only then). For you, once a week may not be enough :-) but it's way too much for him. Maybe every other Thursday evening (or whatever weekday he prefers) could work.
Make this on his terms. You're at his mercy, but that doesn't mean you have to beg and grovel. It just means you might need to accept the scraps he throws at you and be grateful. If you try to be the one to state demands and set the terms, it won't work; if you can make it come from him then chances are better.