• You could donate to the server, help pay the bills and be rewarded.
• Vote (honestly, no cheating), be rewarded as well.
• You can be the person in a community that makes other players want to stay in the community.
• Contribute to the server with more than social challenges, build things, host games, etc..
You can do a lot of things to help keep players, everything you do affects someone somehow.
As Loaf said, Players come and go, it's typical, it's up to the person if they want to stay. However, do expect more players with the upcoming PvP.