Firstly, Congrats. Around half a million users in three months is a great achievement.

But as for 'now what?': I noticed that there is nothing in your list that I would state as being a direct win/win. Almost every item on your list seems like a distraction (yes, a distraction that will get you the really important money).

The exception is your suggestion of 'premium features'. But, if not done correctly, you start competing with yourself - and your free users might start to resent you. Instead, what if you focused on one or more features that are:

(a) free but bring you revenue - like helping users sell albums (and taking a small cut).

(b) require payment because of some limitations - like showcasing a users profile for a day.