I think they will be motivated to join and retake control of their personal online presence. Most people do not solely rely on Facebook to keep in close contact with their family members. It is more of a causal liaison point for posting pictures and such. Imo, changing over to another social network isn't that big of a hurdle for most users, especially when they will be transferring their information not only to a social network, but a social sharing platform which will host not only Synereo, the social network, but also a plethora of other online social software.
Afaics, Facebook is for sharing/interaction/feed amongst strangers, friends and family, with more emphasis on the first two than family.
If the "other online social software" has some compelling features, then they may adopt. I have not yet seen a list of these planned features and an ETA on their implementation.
The technology stack which Synereo utilizes and is built upon is an open source platform which any other type of social interaction software can use and build off of the existing softwares' userbase.