I don't see how the SFF.SE community could decide as a whole whether Pottermore is considered canon. As Slytherincess puts it, it's up to each individual to decide what they consider canon.
From our perspective, you are certainly allowed to cite Pottermore, like you're allowed to cite anything else (the books, the movies, interviews, what some random bloke wrote on his blog, …).
I had a quick look through the site's terms and conditions and saw no non-disclosure agreement. (Note that I did not perform a comprehensive review, and I'm not your lawyer anyway. What do with Pottermore content is your responsibility.) Subscribers agree to “ensure that […] all information provided by [them] via Pottermore is accurate, true and up to date in all respects and at all times and is not misleading in any way” and not to “make commercial use of [user-generated] content”. I didn't see any additional right granted above copyright law, so only fair dealing is permitted; in particular, direct quotes of material posted in Pottermore is likely illegal.
Note that we are not enforcers for Pottermore. If a contributor to Stack Exchange posts material in violation of the Pottermore terms and conditions, it is up to Pottermore to take any action they see fit. We will not edit Stack Exchange content or take sanctions against Stack Exchange users on the basis of Pottermore terms and conditions, since we are not a party to this agreement. If content is posted to Stack Exchange in violation of the copyright of a Pottermore contributor, said contributor can request the removal of the offending material by following Stack Exchange's copyright takedown procedure (§15 of the terms and conditions).