For some of them I try to stay on top of where many of these people are, as it's a hobby of mine to keep up with. For others I either know them, have worked with 'em, or know someone who has / does - so it's easy to track 'em. I only go through reliable resources to keep up with people who I haven't heard about lately.
Also, props to Fugitive for giving good answers, and
@ the 'WATN Tag Team' comment...