The one bit of social media that I use regularly is Instagram, mainly because my middle school daughter has a couple of accounts, and I like to keep track of her and her friends. (Yes, I'm one of THOSE moms). But it's really a lot of fun to see the pictures that she wants to share, and the things that she writes to accompany them, a terrifying insight into the 6th grade mind. My account is personal and private, primarily to amuse my daughter and her young friends, so I post photos of our pets and my daughter. I was on a trip to SLC recently and I posted daily so she could see the weather (60 degrees? Really?) and some interesting places I visited that she's familiar with. Though I didn't post photos of the Family History Library where I spent 3 glorious days, I posted images of restaurants that I visited and local sites. Instagram is an app that you can download to a tablet or smartphone, and I haven't yet tried to figure it out on my laptop but you can access it online as well.
As far as a useful genealogy tool, I can imagine that if you were doing a group family history project, it might be nice to post images of documents that you discover in your research. And what a great way for families to share photos that they have in their collection! I've heard of people using it as a research log, where you could take a picture of a genealogical find, and then add the citation in the notes section. Not exactly the best way to track research, but what ever works for you.
It also a great way to promote your business, though I choose not to use it for that purpose. Similar to a twitter account (I would have to assume that because I don't do twitter), it could be used to update your followers on where you'll be speaking, or something new and interesting that you've found and want to share, and any promotions that might be new and exciting. The biggest drawback to the social media aspect of modern technology, in the mind of a true luddite like myself, is that it takes time to update everything, to be constantly searching for and creating new content, when what I'd really rather be doing is researching and writing! But this might be a quick and easy way to create fun content to share with your followers!