As long as Twitter (and Facebook even more so) don't allow Google to actually index them fully, I don't think that a service like the one outlined here would work very well.

And even once these services open up more: At what point would you set the threshold for inclusion of $social_network in the list? I certainly would not want a list of various social network batches under every search result.

Only list the ones the user is active in? Sure. But how do you determine "active"? Networks in which the user posts a lot? In which he reads a lot of other users content (how would Google find out about that?)?

The integration as it is currently made for G+ basically only works because Google has all that G+ profile data and for it to work as well with other networks, I'd say, Google needs specialized access to it.

Or to be more cynical: So twitter wants to have their accounts integrated into the search results of a search engine? Easy: Just create a search engine too and have all of Google's user switch over.