20. The computer program product of claim 18 , wherein the computing of candidate tags further comprises: retrieving additional tags that other users have applied to the document; and associating an importance score with each additional tag.
20. The computer program product of claim 18 , wherein the computing of candidate tags further comprises: retrieving additional tags that other users have applied to the document; and associating an importance score with each additional tag. 21. The computer program product method of claim 20 , wherein the computing of candidate tags further comprises: retrieving information about the user to whom the additional tags are recommended, from at least one online social system; extracting a set of keywords from the retrieved information; and associating an importance score with each keyword, wherein each keyword is a candidate tag.