Ash is right. In fact, I'm also in touch with people from a field I'm not in anymore, and right now, I'm doing some writing work for them that's the best paid work I've had in a while. And that's setting aside the dinners they took me out for when I was in Tokyo.
When I think of the work I've gotten - and this is in three or four vastly different professions - nearly all of it has been because of who I knew. My first writing gig was because my best friend from high school convinced her colleague to give me a chance. My first zookeeping job was because I'd volunteered for years and they knew me.
Even in academia, which the naive may think is a meritocracy, who I was associated with in grad school was the first thing people cared about. The second thing was my own work. Once you got past those two hurdles, the decision was made based largely on how much they felt like they could stand your personality if you got tenure and they had to work with you for the rest of your life.
So really, no matter what your other skills and accomplishments are, faking being friendly to people is pretty much essential. Even in writing gigs where I have never met the people I work with in person - often never even spoken to them on the phone - I think it has mattered a huge amount that I've learned to stick little bits of friendly in our email interactions.