Why do you use social media? (you are here, so you do - whether you realise it or not!) People use social media for all sorts of different reasons.
Some use it to converse and interact with people that they know very well. Either professionally or socially.
Some people use it to network with mere acquaintances. Some with people they do not know at all. Sometimes peoples motives for using social media aren't apparent.
The reasons I use social media are wide and varied. I use my personal Facebook account to converse and share with family and friends. I also use it to create Groups pages for my personal family history interests. I've had quite a few previously unknown family members and/or fellow researchers contact me as a result.
I manage a professional Facebook account (Auckland Research Centre) I use to promote my specialist subject and collection (family history) and my library.
My personal Facebook account has strict security controls, so that I can control who sees what exactly. The professional account has very few security controls - I want as many people to find and interact with us as possible.
I use my Twitter account (@genebrarian) for the widest reasons: to converse with friends and colleagues, network with professionals, to promote my library and collection and for my learning. There's an amazing amount to be learned by clicking on someone's link. I also follow people "for the hell of it". Their "Tweets" interest and inspire me.
I manage and write for the Kintalk blog for work, and I've just started this blog.
LinkedIn is still relatively new for me, I'm dabbling to see what I can get out of it.
I suspect I can learn alot from it, as well as network.
My personal and professional interests are not just family history and libraries,
they are also graphic design and print (my old career), marketing, digital preservation/digitisation, internet and the web, and yes social media. So I hook up with as many contacts on there as I can.
I also use delicious to store the many hundreds of useful bookmarks I find when trawling the internet. They are portable, I can access them from any computer any time, and I can share them with people - I can "follow" others with similar interests as me, so I can share their bookmarks too.
Its great how you can link with people from one Social Media account to another. Someone you follow on Twitter, you feel you get to know them, so you then feel you can follow them on LinkedIn etc.
I think the Social Media explosion is definitely a boon for us family historians - as evidenced by the amount of blogging (and microblogging) being done out there!