The newest is Mystery Buffs, a Yahoo group at https://groups.yahoo.com/group/mysterybuffs
Make Mine Mystery is a Facebook group at http://www.facebook.com/groups/makeminemystery/
Books Gone Viral, a Facebook group, which has over 10,000 members, is at
Book Place, another Facebook group with over 10,000 members is at
Those are just a start. There are many more I could mention, but I don't want to overwhelm you.
If you wish to add more in the comment section, please do.
and mysteries at
as well as at