This happened a lot more at one time; during the Incursion lead-up events, we of course had Sansha and CONCORD actors happily posting away with a smattering of faction representatives appearing as well. Arek'Jalaan mainly involved a single character, but he was fairly active until the project was cancelled. Finally, during the time when we had a lot of smallscale events, various actors posting (both in official 'announcements threads' for events and just casually responding to player posts) was very common and widespread - I remember characters from all 5 pirate factions, as well as empire navies and various involved corporations.
Going back a bit further, when Empyrean Age was launched there were several objective-based events in the first few months (i.e., capture this system or area in X weeks) that involved IGS threads.
Finally, going way, way back to the days when we had written news stories that weren't only linked to big expansions or huge ingame events, I remember on rare occasion a character being created to respond on the IGS when a particular news story arc REALLY grabbed our attention and garnered a good response. The one that sticks in the mind was the wife of a CONCORD commander seeking information on her husband's disappearance.