Definitely agree with the personalised aspect of the email, but a few other things popped into mind as well. Looking primarily at Facebook, people do not use the site as a place do go to to buy things, whereas it is quite common for products and services to pop up in your email for one reason or another like agreeing to s ign up for third party information sharing, so people are more willing to give this the attention and time compared to glazing over something on Facebook. I think also the way we access Facebook would also influence this as many app versions of the site filter content and may not necessarily show these products and services that may appeal to users. I know I would not be on fb anywhere near as much as I am if I only accessed it via a computer, where as the way emails are viewed is constant between computer and smartphone/tablet etc. so I feel this May influence on the success comparator of email marketing between the two mediums