I totally agree about beauty gurus, its suspicious that they all start talking about the same product at literally the same time and then in the following months they don't mention it at all. I have a question about that. Who sends the products to the beauty gurus? Would it be the marketing department of a company or has a new social media department been created? Sorry if it's a little off topic.
I'm not an insider so my informations may not be totally accurate but I know a 'new' beauty youtuber/blogger and apparently, she sometimes receives products directly from the brand. In her posts, she's quite transparent and says she got the product from the brand for a test but well yes, I suppose a lot of sponsored posts are 'hidden' (just like with fashion bloggers).
It seems there's also some creative/marketing agencies which contact bloggers/youtubers and send them products from random brands. I suppose these agencies are hired by the brands to get the social media coverage without needing to create a whole new department. Or maybe because it's still quite 'controversial' to send products for this kind of 'advertisement'.
As I'm french and talking about french bloggers/youtubers, I think maybe it's not the same in the US or in any other countries. I know that hidden advertisement in France is legally prohibited so brands may need to be careful on how they manage this kind of stuff.