Check the HTML of your email/template. Look for the <img src> tags for the social media icon images--if there is a space between the first quote and the beginning of the URL, delete it.
Wrong: <img src=" http:// . . .
Right: <img src="http:// . . .
I've found that in editing emails and inserting images, sometimes an extra space is inserted by Marketo's email editor. The image still shows up just fine in the editing tool (and in Gmail), but I had the exact same problem with email images in outlook.