Is our commenting on the amusing stances or statements of participants in programs, and thanking them via hashtags—which they might see later and interact with—considered overstepping the boundaries of speaking with non-mahram men or backbiting them?

Allah created humanity with an inclination for the two sexes towards each other. Therefore, Islamic law prohibits communication between men and unrelated women without necessity, and this includes electronic communication. Thus, your communication with men through laughter and joking in electronic comments goes beyond what is permissible, and you should refrain from it. Backbiting is mentioning a Muslim in their absence with something they dislike, as the Prophet, peace and blessings be upon him, said: "Your mentioning your brother with something he dislikes." This includes any mention of a Muslim with something they dislike concerning their body, religion, worldly affairs, or anything else, whether verbally, in writing, or by संकेत. So, if the comments include mentioning an absent Muslim with something they dislike, then it is forbidden backbiting.