I would try and use a couple of methods. Firstly highly targeted ads are pretty much a no brainer. Look at ultimate-guitar.com - they are constantly pushing video lessons etc.

Use the data you have and display the most relevant ads possible a la Facebook.

Secondly, I would consider a low cost premium model. Most musicians are desperate to promote themselves so a few dollars a month to get in front of nearly half a million viewers is good value.

You might find many people are looking for band mates as well, a premium model could make this process easier for them and I think would be well worth a few dollars. With half a million users you don't need to get many premium subscribers to earn a decent living from this.

Finally, I would try and contact some advertisers and maybe even record labels or something directly and see if they have any suggestions. They likely have more experience and will probably bite at your ankles to get some exposure on such a targeted network.

Bottom line, try multiple revenue streams and test, test test!

Good luck!