I know it's largely a one person experiment and not a real business, but some feedback
> I won't sell your data and will be very personal with you.
This isn't good enough anymore. You need to promise that my data won't ever _ever_ be sold. Especially since you're asking me to share my diary with you. I'm not sure if this kind of promise can be made though. Maybe we need some legal apparatus you can declare that gives me peace of mind that no future owner of your company can change their mind.
Although, a web service to share notes with friends is probably okay.
Well that's more of a journal. But yeah, a "diary" service can be used for less private stuff too.
Here're some facts that could help you to find peace of mind. First of all, I operate in the EU, so I can't simply sell your data. Also, it costs me virtually $0 to maintain the service (thanks to Firebase and Mailgun), so I won't be forced to sell out to keep it afloat. At last but not least I use it myself with my close friends so we're in the same boat.