If am not wrong for a while Facebook backlinks do get index by major search engine notably Google, but of late they seem to be back linking them only if you specify for them to do so which just cost you some extra time and effort. On Twitter your back links are index only that they are nofollow links and have no value to your site. Which one do you prefer
I use both, people will have there own biassed opinions of what they believe is greater than the other, but honestly hammer the s**t out of both of them and you will have double the amount of quality links
Both are effective to get direct traffic but I've seen nofollow links to external site in both the websites so According to me they doesn't affect your PR.
Actually it would be had to compare quality traffic, but from the number of visit coming to your blog from these two site is easy to compare.
I do think that they do affect your page ranking because there quality as do not count at all on ranking based on link quality