I believe I read somewhere that the Live Photos saves the photo with sidecar data as one file, but I could be wrong. If that's the case, it's only a matter of the image viewing software adding compatibility to make the images "live". However, if the moving image data is a separate file, then it could be that whenever you share the photo, it will only share the single JPG without the necessary data for the "live" moving data. I'm sure we'll know more when the phone is actually reviewed, but until then, I'm not sure it's clear how it will be handled.
Since they specifically mentioned Facebook adding the capability, I'm sure others will follow. Whether Google Photos will add the capability is unclear, but Google has been very good about iOS compatibility. In the meantime, I suppose we'll have to back up to iCloud AND Google Photos. Hopefully, Google Photos will at least still back up the standard JPG instead of just ignoring (or giving an error) if Live Photos is enabled.