In the team where I work there has been a relatively high turnover of staff whilst I have been there. There are 5 in the team, and over 3.5 years 4 have left. I got on particularly well with 3 of these, better than I get on with the current team, and still keep in regular contact with them (phone calls, emails, and texts.)
Two of them still live locally and we have met up several times over the last 18 months or so. A while back one of my current colleagues found out about this and said she felt hurt, so next time we invited her (and others) along too, and she did join us.
We're meeting up again in a few weeks' time and I don't feel want to invite her. She hasn't been in touch with these people, whereas I count them as my friends.
(I used to organise regular nights out with the whole team, but it was a lot of work, and quite frankly I can't be bothered any longer. If someone else were to organise one I'd be happy to go along.)
AIBU