The trouble is that the desktop metaphor depends on your programs being analogous to physical tools. A text editor is a notepad, a directory is a manila folder,

The trouble is that the desktop metaphor depends on your programs being analogous to physical tools. A text editor is a notepad, a directory is a manila folder, a contacts app is a Rolodex. What iconic tool represents Twitter, or "app store," or "password manager," or "detect the song that's playing right now"? The metaphor breaks down and your icons become mere abstract shapes. If no other meaning is relevant, an eye-catchingly stylized version of the first letter of the service name at least hints at what this is.