Overall engagement rates on Facebook, Twitter and Instagram are around 0.58%. In comparison, the average open rate for emails across all industries is 23%, with an average click-through rate (CTR) of 4%. Most of my clients' open rates are upwards of 40% — almost DOUBLE the average! The newsletter is for all those people NOT on Facebook or Instagram everyday and/or who don't see every post. (I certainly don't, even though I check Facebook daily.)
Newsletters also offer more direct linking to product items. While Facebook does too, Instagram does NOT. Each item in the newsletters / social media campaigns should be able to be purchased online, so we can have clickable BUY NOW buttons and track the effectiveness of the newsletters and social media.
Some Other Interesting Stats re: Newsletters via: