var tasks = workItems.Select(x => Task.Factory.StartNew(() =>
{
    DoSomeWork(x);
})).ToArray();

Task.WaitAll(tasks);

public async void DoSomeWork(WorkItem item)
{
    var results = await GetWorkData();
}

public async Task<List<WorkData>> GetWorkData()
{
    var fbClient = new FacebookClient();
    var task = fbClient.GetTaskAsync("something");
    var fbResults = await task;
};