We post when we have relevant content, which typically is a mix of new products / sales (we are eCommerce) as well as 'interesting' on topic content (funny videos, blog posts, etc from across the web). We also run contents like 'caption this picture' which are very popular

We see a good amount of likes and comments back and forth amongst the customer base, and as mentioned before it drives traffic to our website as well and keeps our 'brand' top of mind.