I have written for you a sample from featureSet json to feature class
string jsonfeatureSet = "{\"displayFieldName\":\"\",\"fieldAliases\":{\"OBJECTID\":\"OBJECTID\",\"Indirizzo\":\"Indirizzo\"},\"geometryType\":\"esriGeometryPoint\",\"spatialReference\":{\"wkid\":102100},\"fields\":[{\"name\":\"OBJECTID\",\"type\":\"esriFieldTypeOID\",\"alias\":\"OBJECTID\"},{\"name\":\"Indirizzo\",\"type\":\"esriFieldTypeString\",\"alias\":\"Indirizzo\",\"length\":50}],\"features\":[" +
"{" +
"\"geometry\": {" +
"\"x\": 940411.3699657875," +
"\"y\": 5643498.120243863," +
"\"spatialReference\": {\"wkid\": 102100}" +
"}," +
"\"attributes\": {" +
"\"OBJECTID\": 1," +
"\"Indirizzo\": \"VIA SORDI, 1\"" +
"}" +
"}]}";
IJSONReader jsonReader = new JSONReaderClass();
jsonReader.ReadFromString(jsonfeatureSet);
IJSONConverterGdb JSONConverterGdb = new JSONConverterGdbClass();
IPropertySet ppOriginalToNewFieldMap;
IRecordSet pRecorset;
JSONConverterGdb.ReadRecordSet(jsonReader, null, null, out pRecorset, out ppOriginalToNewFieldMap);
Type factoryType = Type.GetTypeFromProgID("esriDataSourcesGDB.FileGDBWorkspaceFactory");
IWorkspaceFactory workspaceFactory = (IWorkspaceFactory)Activator.CreateInstance(factoryType);
IWorkspace workspace = workspaceFactory.OpenFromFile(@"C:\Temp\testsd\sd.gdb", 0);
IRecordSet2 recordSet2 = pRecorset as IRecordSet2;
recordSet2.SaveAsTable(workspace, "Test");