The best alternatives to Cluster are Moments by Facebook, PERI Duo Case, and Shuttle. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of Cluster?
.Tech Domains — Building a tech startup? Build it on a .tech domain. On the house for Product Hunt makers
A Social Network focused on connecting people's collections. Every collection gets its own Chatroom for users to easily bounce opinions and ideas off each other.