Sure, I don’t disagree, YouTube indeed has many social features. (By the way, I wasn’t aware of direct messages and groups on YouTube, thanks for pointing that out.)
My point is I would suspect only a fraction of those professionals who reply saying YouTube is an influential medium actually found the content that impressed them through YouTube’s social features or even search.
In other words, I suspect that for most it only serves as a familiar content platform, not a source in its own right. But if you put it in the questionnaire, people will tick the box because of familiarity.
In the end, YouTube may well in many cases still be the right place to host videos (reliable mobile experience etc.), but seeing it compared to not just social media but also news channels like the FT or Politico just doesn’t make sense to me.
(I am talking about public affairs in particular — I appreciate that the situation may be very different when it comes to brand marketing, young audiences and YouTube stars providing endorsement.)