While these "vacant islands," as they're called, are slowly rolling out to all players, when you do receive them, you'll be able to build them by spending energy and collecting a few building ingredients with the help of friends. Vacant islands come in different sizes, but all require four energy to begin. From there, you'll need to ask your friends for Landfill Sand and Landfill Grass to turn each island into a usable "structure." Depending on the size of the island, the required number of materials varies.
1x1 Island - 1 Landfill Sand, 1 Landfill Grass
3x3 Island - 2 Landfill Sand, 2 Landfill Grass
6x6 Island - 4 Landfill Sand, 4 Landfill Grass
While this might not sound like a big deal, actually expanding your city out into these water squares so that you can build these islands is a time consuming endeavor to say the least. Once you've finished an island, you can place any number of buildings on top of them, so long as they're small enough to fit the limited space. Islands can't be deleted entirely, but if you wish to reclaim some water space for decorations or boats, you can store each island in your warehouse for future use. Whatever the case, building these vacant islands will likely be a time consuming process, but that extra land space is probably worth it if your town is simply overrun.
[Via: CityVille Wiki]
Will you build any of these Vacant Islands in CityVille? Do you still have plenty of actual land squares left to expand out to, or do you need this space out in the water? Sound off in the comments!