Facebook logo and guidelines: facebookbrand.com.

Twitter logo and guidelines: twitter.com/logo

Instagram logo: instagram-brand.com
Instagram guidelines: click here

YouTube logo: click here
YouTube guidelines: click here

Vimeo logo and guidelines: vimeo.com/about/brand_guidelines

Vine logo: vine.co/logo

Pinterest logo: business.pinterest.com/brand-guidelines

Snapchat logo: support.snapchat.com/a/ghost-logo-usage
Snapchat guidelines: click here

Tumblr logo: tumblr.com/docs/en/trademark_guidelines