Agreed about promoting on social media. A rap mixing engineer and befriended and approached my band on Twitter the other day with examples of his mixing and mastering. He also took the time to listen to our tracks and pointed out one which I haven’t mixed yet. This is just an example of how you could find more clients.
You need to add lots more to your portfolio. Also list the equipment you use, for example which Digital Audio Workstation are you using for mixing and mastering? List any expensive VSTs you use in the mixing process as this may sway some musicians familiar with mixing to use you if you have a plug-in they don’t have. Which program are you using for auto-tuning (Melodyne, Antares Auto-Tune or Logic Pro X Flex Pitch)?
For your instrumental composing gig, which instruments do you play? Is it just beats you do? What exactly do you provide? Is it a midi or recorded instrument for example. How many instrument tracks can they expect to receive?