I post on a board where we've all known one another for years (decades now) and have met a few people from there. A lot of the members are good 'real life' friends now, too, meeting regularly and going on holiday together etc. Whilst that is a good thing, it does kill off discussions after a while, as people often don't like confiding in people they know well. Over the years I've been part of a few similar 'private' boards and they tend to fold when too many people get to know one another offline.
It's one thing to moan about your husband anonymously, but it can feel disloyal when a few people know him well. Similarly, posting about a work situation can be really helpful to get objective views, but when everyone knows your role and workplace it's just not the same. Also, it is much more difficult for newbies to feel part of things when everyone else knows each other well. In the end, instead of being somewhere like here, where people can discuss relationships or politics openly they become a bit like Facebook, with people talking about their day, or what they've bought recently and how the dog is getting on. It's much more difficult to get new people to join in, so posts slow down and people drift off to where they can have more 'meaty' discussions in private.