The good news is that the Olm end-to-end cryptographic ratchet that Matrix is in the process of deploying (https://matrix.org/git/olm) is built using the same algorithms as Signal Protocol's ratchet (although it's an independent implementation) - so we're hopeful that at least technically the window is open in future for using Matrix to defragment all the services who have adopted Signal Protocol (WhatsApp, Google Allo, FB Messenger, Signal itself etc) without compromising the E2E privacy. Right now this is total sci-fi, and won't likely happen (whilst preserving E2E crypto) without cooperation from FB, Google etc.
However, we hope to get Matrix to the point where they see that the longer term benefits of participating in a healthy open ecosystem outweigh the short term benefits of trying to lock users into a silo - just as email eventually interoperated over the early internet. That's a while off, but this is still the goal.
The best way to make sure this happens is to play with Matrix in its current form, and help us write bridges (e.g. https://github.com/matrix-org/matrix-appservice-bridge/blob/...) to interface as many silos as possible into Matrix. The more bridges, the more useful Matrix is, and the higher the chance of building an ecosystem which eventually Google, FB and friends will find attractive.