The Ombudsman's website has had a makeover - and one of the new features is a guide on Official information requests made by twitter and facebook [PDF]. This tells us what we already knew: a request can be made in any form, including by tweet. If an agency has designated an official account, then they must accept OIA requests made to it, and they are of due particularity and from an eligible requester, then they must be responded to according to the law. The agency may want to seek a physical or email address to actually deliver the information, but they have to respond.
There's also helpful advice to requesters, particularly about the danger of such requests being overlooked. And there's a note which is of relevance to FYI users:
Note that different considerations can come into play when releasing information to the world at large, as opposed to releasing information to a particular requester. For instance, privacy concerns in the former context may be heightened. Therefore, quite apart from any practical considerations, there may be other legitimate reasons why an agency would prefer to make information directly available to a particular requester, rather than post it online for all the world to see.Since the purpose of FYI is precisely to post responses online for the world to see, this effectively means a stronger privacy ground for such requests. But I don't think FYI will have a problem with that at all.