I have used almost all of them under the mistaken impression it enhances my sites aesthetics and the information provided by these widgets are useful for the reader.
Traffic to blog posts, I have noticed, is because of Content.
What we write, and its usefulness to the reader drives traffic.
And authenticity with sources for the information.
Anything more than this distracts the reader.
People browse, in general,for fast and relevant information
There are exceptions where people want to pass time.
For this they have a lot of sites including social networking sites like Facebook,Myspace,Tumblr, thought he last one accommodates serious stuff as well.
One has to choose a Theme which has minimum elements technically yet loads faster.
The Color schemes may have a role in this.