That's rather broad, which metadata are you thinking about? Especially given the sealed sender feature. Assuming you have access to everything at Amazon, what can you deduce about Signal users?
I can think of:
- IP address (you can tell that this IP address sent some Signal message)
- size of messages
- timestamps of messages (when they were received by an Amazon server)
IP address leaks a lot of information but there are still workarounds, and it seems reasonable if you're in a no-trust model (meaning Signal's servers wouldn't be any better than Amazon's). In any case, that's way less information than other mainstream messengers.
On the other hand, one distinguishing feature regarding metadata is groups: group membership is not known by anyone outside of the group if I understand correctly, contrary to WhatsApp (and others).
Not really. Original funding came from NGO sources such as the Open Tech Fund.
+ It usually just works
+ Reasonable desktop experience (needs to re-link once a month or so, but otherwise independent and not terrible UX), good mobile experience
- Metadata handled by Amazon
- Phone number is a hard requirement, and changing your phone number means re-connecting to everyone
- Funding comes from Facebook from what I recall, and even with large amounts of their $100M invested, their expenses are 8 times larger than their income.
+ At least it's a foundation and their finances are not a black box!
~ With a build from an untrusted third party, you can make it work on Androids where Google Play Services are intentionally firewalled off.
~ No audit of the clients. The protocol, sure, but most bugs aren't introduced on a protocol level.
These are only things they could solve, i.e. that others do better. That their contact discovery solution (where you upload your phone book) is broken isn't a downside because nobody else has that figured out either.