The purpose of these icons is to link into the target sites in some way, and as an effect help them boost popularity, which really is what these sites want. So using their icons is really free adverticing for them, and they will have no objections to that. These icons identfy these sites, and these sites only.
It is a total different case with other type of symbols, which describe a more general concept, like e-mail, users, etc. that are designed by someone, and there might be royalties attached to using a special icon image with a unique design.
Here are references to pages from some sites with conditions for use of their brands:
Note that you are not allwed to use the full Facebook logo, only the "f" logo.
I. Twitter Content
2. You may use the Twitter API and Twitter Content in connection with the products or services you provide (your "Service") to search, display, analyze, retrieve, view, and submit information to or on Twitter. You may use the Twitter name or logos and other brand elements that Twitter makes available in order to identify the source of Twitter Content ("Twitter Marks") subject to these Rules.
Note: I didn't find any place here where it stated you have right to use the Dribbble logos, so I'm not sure this is OK here.
3. Intellectual Property
2. Vimeo hereby grants You a limited, non-exclusive license to use the Vimeo Marks for the purpose of publicly displaying and distributing Your Application and allowing End Users to use it.