I get this question quite often. I met a guy yesterday who has had 18000 free download of his mobile app, knocking the doors of angel investor without success
I feel what is needed in the first 6 month is definitely traction but more importantly stickiness.
The metrics that you should be tracking is how many of your early adopters are really using your app, coming back to you every month , giving you feedback (positive or negative feedback) every month.
Once your series A investor see stickiness & feedback from your early adopter you will see flocking towards your venture