We recently launched PubliciTweet which is still in a public/private beta. We have a few hundred users signed up but mainly spammers. They've been proven to be great testers since they are fairly heavy users pushing out lots of messages. It's helped greatly to iron out many kinks in the system.
But now we'd like to shift focus on what we deem to be our target customer. This would be marketing firms or musicians/small businesses.
What do you recommend as a starting point for marketing.
That a side.
You say you want to target musicians / small businesses. Start with musicians.
* Email owners of some of the smaller digital distribution sites. They can only turn you down. Or they could invite you to be their goto on twitter.
* Find out where musicians post music to help them be discovered or simply because they want to. Talk to them, be passionate about your system.
Im not going to sit and list off websites I suggest you approach as it might look a lot like spamming.
Maybe if you explain some of your current approaches and which are working and which are failing. Then maybe the "old crowd" can give pointers. Or equally learn from your mistakes, success.
On a side note, I would lose the spammers.