TogetherWeServed.com has done a great job for military veterans. They break their sites down by service, which makes it difficult if you're trying to reach out to someone from another service, but it's possible to have "guest access" among sites. It's one of the Web's best resources for finding shipmates.
What I appreciate about their websites is the formal database structure. For the Navy version, all of the Navy's military commands had already been entered into a database so that you couldn't screw up the spelling or make up your own commands. Qualifications and other fields were also standardized to greatly enhance the search features. If you could only remember that some guy named Joe was at USS USETAFISH between '83 and '85, if Joe was registered on the site then you'd find him.
I think their most valuable service is a website version of "oral history". They provide the questions for you to answer on your profile, with photos or whatever else you have, and then they randomly feature these memories with other members. They're building a huge database of deckplate memories that should someday be very useful to demographic researchers. It's a personalized military history museum.
TWS was originally founded by a couple Navy veterans, but now they're behaving as if they're trying to scale up for their angel investors:
We would like to announce our recent launch of Police.TogetherWeServed.com (PTWS), a secure, richly featured website specially created for current serving and veteran Law Enforcement Officers to connect with their Brother and Sister Officers across the country, share camaraderie and knowledge, network for professional and social purposes and create a visual record of their law enforcement service in the most accurate and comprehensive presentation existing today.
Police.Togetherweserved.com is run by retired Police Officers for Police Officers. It has no public access and membership is by invitation only.
If you currently serve, or previously served, as a U.S. Police or Federal Officer, State Trooper or Sheriff and would like to join several thousand Police Officers who have already joined PTWS, please click on the link below.
I don't know how they verify police membership, but in the military versions the posers are quickly outed by the rest of the group. If you apply relatively promptly then you'll be a "plankowner" (or whatever police call the first few members) and get some free membership benefits. If you wait too long then you'll be asked to pay for access.
It might duplicate a LinkedIn group. But I find that LinkedIn groups are more about sharing jobs and referrals, while Navy.TogetherWeServed.com is more about sharing memories. I've found shipmates through there that I never would've discovered via Facebook or LinkedIn.
I bet in a few months we'll start seeing appeals to join Firefighters.TogetherWeServed, Doctors.TogetherWeServed, NuclearEngineers.TogetherWeServed, ...