Yes, used tea leaves makes outstanding compost. Finely broken tea can be mixed in directly to soil, and coarser, large tea leaves can either be used as a coarse leaf mulch, or added to a compost pile or heap where they will quickly break down into nutrient-rich soil. Be cautious, however, when using tea bags. Many tea bags are not biodegradable. Some are made of nylon, and others may be made of paper but have plastic sealant used to seal the bags, and may not break down completely. It is safest to either use loose-leaf tea (the more sustainable option), or to break open the tea bags before composting the contents of them.