There is a great discussion going on at the ACRLog about Explaining Authority to students.
The blog post and the comments generally acknowledgment:
1. That the equation Library = Authoritative & Not-Library = Not Authoritative is no longer valid (if it ever was); and
2. That authority may not even be the right metric.
I think librarians are in a unique position to teach students how to evaluate resources so they can select the best resources for the job at hand. One metaphor I have been thinking about using in explaing the evaluation process is that of a journalist and their sources. For some parts of the article (e.g. facts, expert opinion), you want to make sure the source is credible and authoritative, and sticking to standard academic sources is smart. For some parts of the article (illustrative quotes, opinions), a “man on the street” quote might be best, so websites, blogs, even tweets might be a good source.
The bottom line is that judging whether a particular resource is appropriate for the job at hand usually requires reading the resource and exercising judgement. And good scholarly judgment comes from reading lots of stuff, and takes time to develop.
See also: What if libraries stopped selecting?