I am not a lawyer, and this is not legal advice. This analysis is provided for discussion purposes and is primarily useful as a portal to other sources of information. What I say below could be wrong, and I have not consulted an attorney in connection with this post!
This is not authorized copying. That website is not complying with our license.
This is probably not plagiarism, since they are not claiming the posts as their own work.
However, as has been discussed recently, plagiarism and copyright violation are quite different things. And this does not abide by the terms of CC-BY-SA 3.0, under which user-contributed content on Ask Ubuntu is licensed. Consequently, unless you have also published the material under other, more permissive terms, it probably does violate your copyright (and that of the other authors whose works are reproduced there).
To quote part of section 4c of CC-BY-SA 3.0:
If You Distribute, or Publicly Perform the Work or any Adaptations or
Collections, You must, unless a request has been made pursuant to
Section 4(a), keep intact all copyright notices for the Work and
provide, reasonable to the medium or means You are utilizing: (i) the
name of the Original Author (or pseudonym, if applicable) if supplied,
and/or if the Original Author and/or Licensor designate another party
or parties (e.g., a sponsor institute, publishing entity, journal) for
attribution ("Attribution Parties") in Licensor's copyright notice,
terms of service or by other reasonable means, the name of such party
or parties; (ii) the title of the Work if supplied; (iii) to the
extent reasonably practicable, the URI, if any, that Licensor
specifies to be associated with the Work, unless such URI does not
refer to the copyright notice or licensing information for the Work;
and (iv) , consistent with Ssection 3(b), in the case of an
Adaptation, a credit identifying the use of the Work in the Adaptation
(e.g., "French translation of the Work by Original Author," or
"Screenplay based on original Work by Original Author").
Every page on Ask Ubuntu containing questions, answers, or tag wikis specifies the license with a link to it and to the Stack Exchange attribution requirements, so it seems the requirement to include the author-provided URI (in this case, a URL for the post) applies.
Whether or not the Stack Exchange attribution requirements are entirely enforceable, it seems that the license clearly provides for their enforceability in instances where no link to the post is provided. That covers the site in question here, as well as most similar sites.
- Report it here (as Jorge Castro suggested) and hope Stack Exchange pursues it. (You may want to check first to make sure the site isn't already listed there.)
- Since you retain copyright to your works, you may be able to pursue the matter yourself by contacting the site or consulting an attorney. (It's quite understandable if you don't want to do this, especially the latter.)
Again, this is not legal advice. (See top for full disclaimer.)