On Facebook and Twitter, you get advertising that YOU do not control. On your self-hosted blog, you can choose NOT to run ANY advertising.
On Facebook and Twitter, every little scrap of date is collected and stored forever to sell to advertisers. On your self-hosted blog, you can be as anonymous as you want to be. And you can choose the same option for readers.
If you get ticked off at Facebook or Twitter, you do NOT have the option of moving all your content to the other Facebook or Twitter because there IS no other Facebook or Twitter. If you get ticked at the company hosting your blog, you can pack up and move your entire site to a different host.
Facebook and Twitter can pull your post, or content for that matter, because one person complains about it. Not so with a self-hosted blog. Because if they try that, you can up and move your entire site.
On Facebook and Twitter you do not own your content. Not yours. On your self-hosted blog, you do. If you are a citizen journalist, you are very concerned about corporate ownership of the media. Facebook and Twitter are corporations.
You can still get sued over the stuff you post on Facebook or Twitter. Same as a self-hosted site. Because Facebook and Twitter have money, your posts are THAT MUCH more likely to attract lawsuits.
You can still use Facebook and Twitter to promote your site. Just post links to articles on your self-hosted in status updates and tweets.
Kim Kardashian and Paris Hilton can sell mentions on Twitter for thousands of dollars. You cannot. You can sell advertisements and collect donations on your self-hosted site.