Sharing Etiquette

I was wondering what the current sharing etiquette is. I share with as many friends as possible every day, and I don't share anything if I don't have something that's new to them. Usually this gets me about 15-25 shares a day as some friends have already been given a card by the time I first open the app for the day. In return I usually have someone give me a card every 1-2 weeks, and I have a fair number of missing cards in the 1-4 diamond range. Is the share function dying, or are all my friends just greedy? Am I only supposed to share if someone shared with me first?