This is a tough one to execute well when thinking about privacy.
The biggest problem with this is what doing so implies: in order to match your address book to users with Mondo, the app would need to upload your whole address book to Mondo’s servers to find matches. That’s pretty scary.
Open Whisper Systems (the team behind Signal) wrote a great post contemplating some technical means to get around this (hashing, bloom filters, etc.), but ultimately it’s not clear that there’s a good solution that doesn’t entail significant trade-offs.
The solution we settled on for now is to only look up the user you select. This gives you excellent control of your address book data, but at the user experience cost you describe above.
Ultimately, we need to find the right balance between shipping amazing features, and respecting and protecting the privacy and personal data of all of our customers.