Engagement vs. virality in web app

A web app of mine is the early stage of development. It has less than 100 users in total. On a daily basis there are new users coming in but relatively less returning users.

My question is about breath or depth.

Should I put more focus in

• creating more engagement with the few existing users (depth) through in-depth feature development, or...
• creating more virality that users have more reasons to spread it to their friends?

I put this as a binary question because one is building quantity while another is building quality. In an alpha phase, which do you think would help me progress?

Let me reinforce the comments from Erik and vnchopra:
You need to get Getting feedback form the existing user base, to understand why they come back, and why they don't.

This will give you an indication of what you need to focus on to increase retention.

When you got that sorted out, and you are "sure" that your users keep coming back, you can then focus on bringing more people in.

But you need to do it in this sequence! Get Feedback->improve Retention->Grow User Base.
It will be useless to invest in getting new users if they don't return after a couple of visits.

I like your way of approaching the problem. In my opinion I would try to get as much feed back as much as possible from the current number of users. 100 seems to be a very good number, and great feed back would then supplement your improvement on your iphone application, effectively giving your users exactly what they want. This also makes it seem like as if they are part of the overall development of the app making it special to them and helping you spread it out to their friends. I think thats how to build a great App.

It sounds like you have an issue with user retention? How may people are actually returning?

I'd ask the following questions:

1. Is it easy to sign-up? Judging by your questions it seems that it's easy for users to get signed up. So therefore they are not returning for a couple reasons, either you haven't made the app fit into their daily work-flow or the benefit they receive is short term, no reason to return?
2. Why would they come back? Are the features such that there is a benefit from returning daily? If so then I would say focus on spreading the app with more breadth. Keep the features simple and appeal to lots of users.
3. Are users suffering from take home syndrome? Is the app worse then the website suggests? ie: once they sign-up perhaps they don't find the app all that useful? If they find it useful then they should be returning?

I would personally focus on the users I have now. Why are they here? What makes it worthwhile them coming back and in time more users will follow. If you try and appeal to the masses you might miss your mark. Ask yourself if there is benefit from the users incorporating this into their daily work-flow, is the content good enough motivation for them to return daily?