I have an iPhone so I can upload photos straight to Facebook. No hunting for cables to download onto my computer first.
There is much more immediate feedback with the like/comment functionality. Instant gratification keeps me going. Often blog posts will be read but if no one comments is it really a conversation or just an internal dialogue?
More of my friends are on Facebook, and they get alerts to new content immediately. It can actually keep me in touch with friends on a daily basis, where you wouldn't necessarily send a text or make a call.
Some of the groups that I belong to on Facebook as specific support groups for Edwards Syndrome. I tend to post a lot in there because there is a mutual sharing of information and support. I read those threads regularly for the same reason.
Other groups are specific to scrapbooking, so you can have those conversations with people that understand your (addictive) hobby and are of the same ilk. In the UK it's not a hobby with as many interested in it as cardmaking might be. There is a cross over in skill set and product used, but they are 2 distinct activities.
All that said, I still try to update my blog. It is a space where I can ramble on about my thoughts, observations and feelings. Its not just a tagline against a photo, or answering a question. So for me, there is a purpose and a place for both Facebook and blogging.
Do you have a preference?