I think this must be true of Twitter in specific places (USA) or circles. I find it to be largely positive and can't think of the last time I saw a polarised debate on there. Not on FB so can't comment about that.
Instagram is important for one of my lines of work and I find it a positive place.
There might just be a lag post-adoption in a place, or require some initiating event. It was great in the US for years until around 2008 when people who are professionally political got on for the election. Until then it was mostly hobby talk, like a bunch of small subreddits that occasionally talked to each other.
We have the professionally political on Twitter in Australia and they're very active, but we don't seem to have the extreme polarisation in politics at workplaces. I suspect that's part of it. Our political differences here don't seem as dramatic.
Instagram is important for one of my lines of work and I find it a positive place.