It can be tricky depending on your location. However, having been through this, it's much easier if you are looking to join an existing nanny share: People get attached to their nannies, and don't want to let them go when the other share family leaves (which happens, owing to people moving or getting into a day care); and those who are looking to fill that vacancy often have slightly older children like you.
Again, though, it depends on your location. To be honest, I have always had *much* more luck with my neighborhood listservs than I ever have with this forum. Because families are generally looking for shares in their immediate neighborhood, the listservs have more relevant activity.