We've tried similar things before. There is a group on yahoo groups already, it's about 80% retired people there though. They tried to establish a community on facebook (watching IBM) and it's an activist site (anti IBM mgmt) - It feels like most folks posting there are ex-IBMers, many of them dislike IBM.
Finally, there is Reddit - folks started a good subredit r/IBM but corporate commonucation folks took over and it turned out into a marketing tool for IBM (mods were IBM comms people) - so, a bunch of us got p-ss-d off and a new subredit was created r/newIBM - it was very good for a while but its losing steam.
Then there are groups on Blind and FishBowl, most of them are new folks or trying to get into the game - there is a lot of bragging about the total comp and other childish stuff - there is a ton of visa/immigration questions, etc. Overall, I do not even bother - waste on time.
Then you have this site which is like a cocroach, resilient and posts for whatever reason never stop here - I think it's a scale thing, peple switch companies but keep coming here as each company has a page. this page is now a bit more than a layoff thing, i just wish they had a section for registered folks so we can network and i'd pay a few bucks if they can turn off ads for me.
i like your idea and i'd join your google group but as i said before, it's an uphill battle to heard the cats.
good luck and thanks for doing this