A combined list works well as all the choices can be selected in a single view. However, users may want choose the network to use so I would supplement this by identifying the network each contact will be using. Another thing to be wary of is that the merged lists may be exhaustively long and littered with multiple duplicates (contacts that exist in multiple networks).
A separate list to select from imparts additional steps if the user wishes to choose contacts that reside in different networks. But may help the user with searching as it acts as a filter. Users would most likely know where contact A can be found, and wouldn't have to sift through a large merged list.
Side note: the app can acquire permission for the phone contacts upon installation. However for Facebook and Twitter they require the user to do it within the app. Unsure if you plan to ask/require from the start, but I feel that the separate list layout may be better suited to persuade the user into adding their social network contacts.