So my whole career in this industry is helping companies 'get open' and embrace 'open'... and I can tell you this is not what will kill Facebook.
Being 'open' isn't a feature consumers directly care about. The teenager on the street corner, the mom in the grocery store, the middle-aged office worker... they are not going to use Service X over Facebook "because it is open".
That means nothing to them. There's nothing in there which explains to a normal person why there is any problem in making their tweets, likes, groups, etc on Facebook.
Further more, disparate services which rely on users having their own control of their node (Dispora, DiSo) make no sense when 99% of most normal people have no web hosting or other way of hosting such a node.
If there's one thing we've learnt from recent Facebook debacles, it is that people appreciate a level of privacy and control around their profile data. Now, open doesn't equate to no privacy - but you have to expect and assume that all nodes will treat privacy the same, otherwise the system fails. With a closed system you simply have to trust one actor with your privacy - the host network.
Finally, the 'dartboard of advertising' is what gives companies like Facebook the resource to outcompete their rivals. If an alternative service doesn't have that kind of advertising strategy then it's going to find it hard to compete with the might of FB.
Open may form part of the ultimate successor to Facebook, but it is just as likely to be something totally closed as Facebook. Open/closed is [sadly] not what consumers care about.