I started my personal blog almost eight years ago to compel myself to write more. It could be about anything, as long as I wrote, and...I've found it's led to greater productivity.
Blogs are very flexible in terms of content. If you're concerned someone might steal or otherwise exploit your ideas, you don't have to put them on your blog. My blog has been mostly been about what's on my mind before I get down to fiction or poetry—in other words, I blog about stuff that doesn't fit either genre: book reviews, TV reviews, movie reviews...
[A]t the very least, a blog is a place where you can control how you and your work are perceived. There's no harm in having one more place to publicize where you and your work will be appearing.
I'm not on Facebook because a blog is enough for me. I am on Twitter because it seems lower-maintenance than Facebook. Again, I use Twitter for random thoughts that seem to have no place in my other writing. I also use it to publicize my writing.
As with anything of this sort, it pays to keep in mind what to put in and what to leave out. Good luck.