An excellent question, Andrew.
There have clearly been returns on investment for making use of web-sites, and electronically available information and forms. Has social media added value above and beyond that? Personally, I haven’t seen it, but that may be more a matter of the visible impact paling in comparison to simple use of websites, rather than having NO impact at all.
I think it is also fair to suggest that the line between government social media, or government use of social media services, and general use of social media, is fairly blurry. For instance, if one simply wants to get some information “out there” in a hurry, it wouldn’t take long, or too many steps in transmission, before what government has put out simply becomes part of the broader grapevine.
The bigger challenge is whether what comes into government via social media (as opposed to what they send out, which isn’t really all that different from a simple website with more push) is used, or even could be used.
Or have I misunderstood the question?