That is quite some hefty code to look through, but the first thing that stands out is that the assignment says that if the V-Logger has no followers, then you should print out only the first line: "(in case the vlogger has no followers, print just the first line, which is described below)". Your code otherwise shows that you can correctly manipulate collections, so try to implement some private-helper methods to sort out the final collections and output-result, example: 

        static Dictionary<string, List<string>> SortVlog(Dictionary<string, HashSet<string>> dictFollowers)
        {
            dictFollowers = dictFollowers
                .OrderByDescending(x => x.Value.Count)
                .ToDictionary(key => key.Key, value => value.Value);

            foreach (var (member, followers) in dictFollowers)
            {
                dictFollowers[member].Sort();
            }

            return dictFollowers;
        }