While there are many options for this, I regard it is too time-consuming for the value it provides. (Also I generally have to visit their sites to keep up - I much prefer to select push systems - such as email groups.) However, I am on three platforms, primarily to receive information.
A bit like Facebook - but for business. So if you are in business (or target business) you should be on LinkedIn. there's not much more to say than that really.
I do not like Facebook - but then I don't like Google, Microsoft or IBM most of the time. Large companies, and in particular large technology companies, are not our friends. (Can you tell I enjoyed "The Matrix".) I found recent US and UK articles on executives protecting their children from their own products quite scary.
FB is, for better or worse, part of most people's lives - and the only way to catch up with some. I was about to give up on it when my son introduced me to Social Fixer. It doesn't solve some bigger problems - including them becoming arbiters of many issues, but it does make it a far better experience than their own settings.
This is a different social media platform, designed to recreate the neighbourly atmosphere so often missing in today's culture. To be a member, you have to be identified with a particular address. In our case, we're a very small suburb, but we can choose to see several nearby neighbourhoods - not the whole of Lower Hutt, but a good part of it. They also have Hutt-wide groups (e.g. for elections). With everyone a real person associated with a physical address (which can be hidden but is still there), there tends to be less of the stuff that puts people off Facebook. It's a good place to share what's happening in the neighbourhood.