Showing the domain of a URL
You can use tuple unpacking/list unpacking to write :

if sites['twitter'][0] in url:
    link, brand = sites['twitter']
elif sites['facebook'][0] in url:
    link, brand = sites['facebook']
else:
    link, brand = sites['pininterest']
return link, brand


Now, the data structure you are using is a bit akward because you don't use the key at all. Instead of mapping string to list of strings, you could have a list of list of strings. Even better, you could use list of tuple of strings :

sites = [
    ('http://www.twitter.com/', 'twitter'),
    ('http://www.facebook.com/', 'facebook'),
    ('http://www.pininterest.com/', 'pininterest')
]

if sites[0][0] in url:
    link, brand = sites[0]
elif sites[1][0] in url:
    link, brand = sites[1]
else:
    link, brand = sites[2]
return link, brand


Now, this seems to be begging to be written with a loop

for (link, brand) in sites:
    if link in url:
        return link, brand
return None # or anything you like