I think you're right about the Fi thing. I developed mine around high school too, nowadays I mostly stay with my room. My environment plays a big part of it.. my campus is notorious for fostering shallow, dumb, rich and snobby kids, and I have no interest at all in talking to them. The friends that I have made are a small group, and they either want to go out drinking every week or sit around and talk all day.. which can be nice but I wanna get up and go do somethin! Sigh, being around IP's all the time can be frustrating..
That sounds about right!
When I get the right people, the right conversation, it energises me incredibly - there's nothing quite like it! But for the most part I am very passively social - I very rarely organise something myself, but I will only ever turn your invitation down if I'm already busy with something else important. ^^
As far as when I'm actually out, socialising? I am not a big talker (unless we've hit that right conversation above), and I always feel slightly guilty if I steer the conversation at all towards something I'm interested in. Besides, most other people are more than happy to take the lead on that one - so I let them.
And especially when it's a group situation, my lack of desire to put in imput that really has no interest for me, often results in my sitting back and enjoying the conversation without much imput at all... there have been times when I have suddenly realised that after 20 minutes of conversation I have said approximately nothing, and am not actually involved at all, even though I felt like I was.
And definitely, some types energise me more than others - my INTP friend and I frequently hit topics of intense interest to both of us, which equals great, engaging conversation. My ENTP friend and I bounce weird thoughts and random tidbits off each other and spend the whole time laughing.
But some other types, it's just a miss with conversation. We don't quite get each other, and so I don't say anything, and we don't talk much at all.