Geologists/environmental folks, do you use social networking sites to keep in touch with colleagues? And how do you use them?
I do use linkedin, as I alluded to in this post when I was trying to find old classmates. I keep facebook strictly personal, although I may have an odd ex-coworker/close friend there. I've also been invited into ResearchGate, but I'm not an academic and I have a hard enough time keeping track of anyone on the two social media sites I do have accounts with.
I think this varies more by personality than by industry, but in my case I keep my contact list relatively small. No relatives or spouses of coworkers (sigh) in entirely unrelated industries. I also don't accept requests from people I don't know who are just keeping a giant pile of contacts for their own purposes. The worst offenders for these seem to be recruiters and salespeople who are trying to get buyers for generic industry stuff (labs, material suppliers).
I accept requests from all coworkers and former colleagues (academic or industry) without question. Same thing with clients, although in one case I ended up putting someone on hold for a while while I beefed up my profile so it reflected something of my experience with that type of project. In the past, I've been hesitant to accept requests from subcontractors, but I've since relented. As it turns out, most of the subs I interact with on a daily basis aren't networking on linkedin (with me, anyway), which makes sense - they already know my full contact info, and if I leave, they can find where I went in about six seconds anyway. My corner of the environmental consulting world is very small.
As I was writing this, I checked to see how many contacts I actually had - about 100 on linkedin. Once I was actually there, I started poking around. Of the former work colleagues, a few people have struck out on their own and formed their own companies. Probably half are working for different firms than when we worked together, but I think that percentage is relatively high because the people on linkedin are more likely to be mobile than those who are not. Several are actively job-hunting. One has grown an epic beard and become a farmer. So, a mixed bunch.