There isn't enough attention to go around.
There is a finite number of people on the planet and an increasing number community projects. In some sectors we are already hitting the limit for possible communities.
So what happens when attention (to participate in a community) is limited and the competition is increasing?
You have to fight for every member.
This is a guiding philosophy you should implement today. Forget going big and sending out mass messages to huge numbers of people. Instead, focus on fighting for every member. Target individual people at a time, persuade them to join your community, take the time to build relationships with them and convert them into active members.
If a member drifts away, fight to bring them back. Ask them how they are, let me know of new things they can do, introduce them to others. Make sure every community member makes friends with others. Take time to individually connect members to each other. Host events where they can meet. Take the time to build strong connections.
Make leaving your community a choice no member would consider making. If you build these strong connections today, no-one will be able to steal your members tomorrow.
I don't think this is a choice. If you're launching a communtiy from scratch today, in a competitive industry, you're going to have to fight for every member. That may not apply to all of us right now, but soon it will do. Don't take any member for granted.
Starting fighting for every member today.