Rather than asking whether posting the same content on multiple platforms would be considered plagiarism, the better question is whether or not it would benefit you to do so. The easy answer to that is no, because posting the same content on more than one platform dilutes its authority between all of the platforms it is posted on. Provided that it is clear the specific posts on all of the platforms belong to your company, most search engine algorithms will have no way to discern which website/platform to serve for this content. In this scenario, it is possible that your content will not be served in search results altogether, on any of the platforms.
A better solution is to syndicate your content across all platforms and point it to one place, such as your blog or website. This way the algo will not detect a conflict where the same content is posted, for example, on Facebook and Tumblr. Publishing the content on one strong and powerful platform maximizes your chances of attaining high rankings for the corresponding keyword searches. At that point all you need to do is make sure that this content ultimately links back to your source of revenue (such as your website or online store).