All Facebook friends are now automatically neighbors. This feature is a Facebook change. Neighbors cannot be removed from the game without deleting them as a friend on Facebook.
After deleting a friend on Facebook, they may still appear as a neighbor. You may choose to block them or wait as it takes a while for Facebook's servers to communicate to Meteor Games servers.
There is a limit to the number of neighbors a player can have, but the number is very large. Also if you have a lot of neighbors the game will only load 400 of them. The 400 you see are randomly chosen so if you are looking for a particular neighbor you might have to reload the game. You can also ask that neighbor for their Avatar ID Number so you can visit their island using a direct link.
Inactive neighbors (sometimes simply referred to as 'inactives') are people who no longer play Island Paradise.
Signs of Inactivity
What you can still do
You can revive their crops, steal from their crops, trees, and animals so long as they have something to steal. If they have not completely removed themselves from the application, you can still send them gifts although they probably won't send anything back.
Getting rid of them
There is no way to get rid of inactive neighbors currently, although there have been talk of a 'truce'.
Bringing them back into the game
Some neighbors will come back to the game if they continuously receive notifications that people have stolen from them.
To become neighbors with someone, you must first be their friend on Facebook. Then one of the two people will send a request to each other and the other accepts. You are allowed to send 24 neighbor requests a day, and this number is shared with the number of gifts you may send.
Alternatively, one can make a post to the live feed which your neighbor can click to become neighbors with.
This was added on November 19, 2009. Every day, you have a chance to post this on your wall and save you the time of sending neighbor requests.
If two people are not yet neighbors, one can send a gift to the other. If the other person accepts, they will become neighbors.