I never did with my eldest, because we knew her group of friends so well and she's extremely sensible and kind of an open book anyway.
With my older son, who has always had a huge social network, I checked a few times each year in middle school and was glad I did. Nothing truly alarming, but we had some good conversations about acceptable and unacceptable ways to talk about girls, the downsides of popularity, and (especially and frequently) why you should never put in writing anything that you'd be embarrassed for your parents, coaches, and teachers to see. We stopped checking texts in HS, though I now follow him on Instagram, and get reports from my younger extended family members about his snap chats.