Make some synchronous social/chat or coworking times available, such as online chat, videocalls, livestreams or open Jitsi/Zoom videocalls. Consider a more low-key structure such as "let's all cowork together", or set up a recurring stream (YouTube/Twitch) where you work on the project, show your environment setup and take questions. Or lead a more discussion-centric "office hours"/"open hours" chat, which can be "about anything" or carry a loose agenda such as "questions about the recent release". (This can also help you constructively funnel discussion of a particular current controversy.) This helps contributors who are interested in friendship grow into friends through increased conversation, proximity, and consistent interaction. Contributors with social ties to a project are more likely to stick around.