Congrats! This is really awesome. You've found some amount of product-market fit, and that is really really amazing. I think you should keep going, as this is when things start to get hard. You've cleared a few hurdles, and now you need to clear some very different obstacles.
From the countless hours that I've spent trying to figure out how to get users to give me money, I can confidently say that a surveying your users about monetization is the worst data you will ever see.
I can also say, with 200% confidence, that ads are not the right decision at this point. Here is why:
1) They aren't your users. These are FB users, and if you start showing ads to them, FB will view this as stealing ad space from FB. Not a good spot to be in.
2) The geo distribution seems pretty clear that the light weight solution is a compelling feature. Ad SDKs are notoriously heavy on the binary, sometimes 20MB. Ads also use a large amount of data and are often slower than everything else. Most web pages could load in a fraction of time if they dropped the ads. So, putting in ads seems to kill the biggest feature of your app.
I don't buy the argument that you can't monetize that geo distribution. It's true that those geos are overlooked by larger companies, but that also means less competition. Just know that you're fighting for low ARPU users. Whatsapp used this strategy wonderfully. When their competitors were fighting over iPhone penetration in the US, they were building for feature phones in similar geos. They also kept the price point low, keeping in mind they had lower ARPU users.
I suspect that your best solution would be to find a compelling in-app-purchase (IAP). To do this, you'll need to start knowing everything about your users. What features do they use? What features do they not use? When do they use the app? What do they want to accomplish with the app? Remember, Line started selling stickers when everyone could use emotiacons. Sometimes the winning business model is the thing that no one thought to try or everyone thought was too dumb. What is FB missing from the experience that you could add? I've always been impressed at how user's actions can be the opposite of what they say. Your users will reveal their intent by what they do, so watch them very carefully.
I suspect a very telling analysis would be to start comparing users who hit the day-7 mark and leave vs those who hit the day-7 mark and stay. My guess is that you'll start seeing some patterns about how retained users are interacting with the app, and then double down on that.
As a side note, your DAU to installs ratio seems to imply poor retention for a social app. This may be because the growth is targeting the wrong type of user. Regardless, this is a number that you'll probably want to get to know very well. If you do have lower-then-average retention, find a way to turn it into a strength with other numbers that are really impressive, such as engagement.
This is really, really awesome and you should be proud of what you have accomplished. The next stage of growth will be harder to scale, but the bragging rights are a few orders of magnitude higher up there. Keep it up!