There seems no recognized or fixed rule for citation sources, as far as I know, the citation taken from publications or other open sources that can prove they are not something groundless is acceptable because at least the citation to be used as support for your argument in the paper has been used or established by other authors and the quality is guaranteed by the publishing houses or agencies like that. Now with more and more information and materials in social media outlets and self-media available, the authenticity of the information cannot be ensured(but sometimes you have to admit there are so many anonymous genius online who are much superior to the counterparts in real life),and will it be OK to select such to be citation?
Always cite a source, but also always make an effort to cite a credible source. So if you learned something through a tweet, still try to track down a (scientific, peer reviewed) publication and then cite this. Only in the case where you are sure (read: did your best) that there is no publication that contains the claim/fact/result you are using, cite something else.