You'd probably find that there are different types of PhD students--the ones that are always hanging around, chatting with lecturers, maximising the facilities, going to every seminar on offer, and the ones who have busy lives outside of their PhDs, and just come in every once in a while for supervisions, an odd lecture here or there. There's no set way. You won't be looked down upon for not getting involved, if that's what you're worried about.

I started out with this same fear. I had moved to the uni from a different city and hadn't made any friends in my department. Most of the students were much younger than me, and I didn't find anyone that I felt particularly comfortable with. I also lived (and still do) about 45 mins away from uni, which made socialising pretty difficult. I'm still in the same boat really, though I have made a few friends and have consciously decided to not let my PhD rule me, which is a personal choice--I understand if people want to make it the centre of their lives. For myself, however, I was starting to find that having other things in my life helped to eleviate my worries. Though you're not getting involved in uni stuff, are you doing anything outside of the PhD eddi?

You are typical PhD!

If you start mingle again, you won't enjoy it as much as the before-PhD days - you would be thinking about that unfinished homework/Probalem sets, you would be worried to death about the next test...

Frankly, you cannot waste your time doing anything apart from study - unless your extracurricular activities helps you to stay alive and healthy in your program.

I totally disagree with the assertion that you 'cannot waste your time doing anything apart from study'. I think this is an unhealthy approach which causes major issues for people (i.e. burnout). I've seen it happen so many times. You cannot sustain this for 3-4 years without running into problems. How are you meant to refresh your thinking about your project if you never walk away from it? It's that proverbial notion of having to step back from something to really see it properly. I think your work actually benefits from breaks, because you return to it with a new perspective and from a different angle.