My main experience and success has been with advertising, so that's what I know, and what I recommend as being easiest to start, easiest to scale.
I disagree with your argument.
>> "it'll take a large audience before you can make any money from ads"
It'll take a targeted audience before you can make money from ads. It's not about raw numbers, but about how well you match your users up to what they want.
You can make a living out of a website that gets 100 visitors a month, if those 100 visitors are extremely targeted visitors.
* Make it more of a reminder service for specific dates
* Make it email/sms/etc you at pre determined times
* Make those communications include handy adverts
For example, "I need to buy a present for a thomas who is 8 by Aug 8th at the latest"... on Aug 1st it emails you saying "Don't forget to buy a present! Here's some cool offers, here's a coupon to buy toys. Here's a great place to buy e-cards, etc etc"
I'd start by analyzing what sort of events/items your userbase are adding on your app, and working out how you can enhance their experience using advertising.
If it turned out that 90% of your userbase are adding the todo item "Find a good deal on car insurance", then obviously it'd pay to find some good car insurance affiliate programs... (Hyperbole example, I know, just to show a point).
But 100 users using a todo webapp aren't really that targeted.
By targeted I'm meaning more like "They have already expressed a desire to buy X" etc
I disagree with your argument.
>> "it'll take a large audience before you can make any money from ads"
It'll take a targeted audience before you can make money from ads. It's not about raw numbers, but about how well you match your users up to what they want.
You can make a living out of a website that gets 100 visitors a month, if those 100 visitors are extremely targeted visitors.