Loads of icon packs have customised versions of the social media icons. IANAL, but my reading of the situation is that the companies hold the copyright for the

Loads of icon packs have customised versions of the social media icons. IANAL, but my reading of the situation is that the companies hold the copyright for the logos, but in general allow people to use the logos, and their own versions of them, to link to their services. So, for instance, if someone were to try to use a version of the Dropbox logo as the icon for their own software or service then they would be sued, but if they use the icon for a link to share something on Dropbox then that is alright because it is promoting their business.