My standard answer to friends and colleagues... "They were taking up too much of my time".
But maybe another reason is more technical. I was using both as advocacy tools. I was putting up various human rights related content.
But it wasn't working. Both systems were "closed"; postings were not searchable via internet search engines. The search functions in both systems didn't do any better. I was constantly trying to find "that post" which I had highlighted something important.
For a while, I tried a hybrid solution. Post on Blogger, and a plugin allowed those posts to appear in Facebook. But it still wasn't an elegant system...
Hence the decision to remove myself from Facebook and Twitter. 2 months on, and I must say that I enjoy time away from them.