Personally, I find these links to well-known sites distracting more than helpful.
- They're in a different color, so they pull my focus
- They make posts "feel" kind of spammy
- I click on them accidentally more often than I do on purpose
- They make the more useful, non-obvious links stand out less
(I also do not like edits linking to a user's Academia.SE page whenever they are mentioned by username in a question or answer, or linking mentions to other answers on the same page, for the same reasons.)
I brought up this issue on User Experience Stack Exchange.
As an answer there points out,
Do you think users have a need to visit Facebook while reading your site?
I do not think we generally want users to click on these links while reading Academia.SE posts (unlike, say, links to useful outside resources, which we do want users to click on), so they shouldn't be clickable.
Another answer advises,
I found little empirical data for UX hyperlinking best practices (couldn't link to it anyway), but find the practice of gratuitous links to everything, including well-known sites, to be annoying, distracting, confusing, and to serve little purpose.
Unless you're linking to specific, relevant information or citing a source from that well-known site, there's no need for a hyperlink. If you're using the internet and are over the age of 12 you know what Facebook is.