A good way to find active friends is to check who on your Friend’s list is very active and has the highest Tavern; tablecloth/seats. Cull your list of inactives and send all of the folks in their Taverns, friend requests. If your list is filed with that many inactive players, I would cull once a week for a few weeks.
After a while, folks doing the same thing will start sending you friend requests and you will have your FL full. I accept any and all that send requests, even those just starting a city. Once a month I go through and cull any that are not active. Make sure to keep up emptying your tavern also!! I do mine a few times a day and if needed, a partial empty before bed, as it will be full by morning. Also, while you are building your list, go through your friends list and check for Taverns that are available to visit several times a day. Their Taverns fill fast also and they empty several times a day. It may be fulI now, but in an hour, empty. The more you visit them, the more they will visit you. I know several folks who only visit by clicking on whomever is in their Tavern at the time, not from the list.
Those quests//challenges with anything Tavern are the easiest ones to complete, same with spending tavern silver.