When should I start talking to my kid about using social sites and apps responsibly?

I believe you should have the discussion when you began allowing use.We covered what's appropriate and what's not and its a constant discussion especially during our weekly family meetings. We also share with our children true stories of how social media ruined someone's life from one simple post or one simple decision to exchange a provocative photo, and also how social media displays your character and can also affect even more aspects in your life as you age for instance employment. Having four teens my youngest is 13 oldest 17 it has worked. The deal is to be as involved as possible. My husband does not use social media but I do so guess what mommy is your first friend, which means I am front row of what their posting. But don't go into friending your children as the parent that wants to spy (in their eyes). My kids took it as how cool my mommys on Facebook, Snapchat, and Instagram. So if your allowing them to use social media a bit early they will really think its cool your their friend and it will put them in the habit of posting respectful material, but one tip as they grow older into their teens there will be postings that your not fond of that a teen post, but I see it as if its not disrespectful, provocative, or sexual why flip some if their postings also give you a bit of incite of their personal thoughts and feelings. If your involved every story doesn't have to end in some of the social media nightmare that we all have seen. So far of them being in social media since my oldest was twelve it has worked for us.