That's just personal/professional networking and you should consider working at it. I try to keep in touch with as many former co-workers as I can in case I find myself looking for something new or I can help out someone who is looking themselves.
That's my favorite opportunity: helping someone find a job when they need one.
You do realize this is the best way to get a job? And also, commonly thought as the best way to find new high quality employees so that when your old colleagues boss
It's not some secret fraternity. All you need to do is to do a good job and not be an asshole, so that when your old colleagues boss asks, does he know any candidates for a new position he remembers to mention you.
That's my favorite opportunity: helping someone find a job when they need one.