"Ask Boris is the latest in our series of Talk London events, your chance to talk to the Mayor about what matters most to you," suggested one of Boris's marketing whiz kids at City Hall. "Join us on Twitter to talk about what makes London the best big city in the world."
And so they did. In their thousands, tweeters took to the network to ask Johnson such vital questions as "Does it annoy you that Custard Creams from Tesco are normally all broken?" and: "Which would you prefer? To be attacked by a dozen duck sized horses or attacked by one horse sized duck?"
The Mayor might have expected abuse, but instead he faced a series of increasingly surreal questions: "Time flies like an arrow, whereas fruit flies like a banana - discuss/comment?" asked one concerned follower. "Did Bono finally find what he was looking for?" asked another.
The session quickly got out of hand. "Boris Johnson is doing a Twitter Q&A under #askboris and naturally some people aren't taking it seriously. Have a look," suggested one user. And before Boris could properly get to grips with the important topic of "how many chucks can a woodchuck chuck" or decide between Curly Wurly bars and fudge fingers, #askBoris was one of the top trending topics in the world.
Whilst previous sessions had lasted a whole hour, the Mayor made a swift exit at half time citing previously unmentioned "diary commitments". Meanwhile most of the genuine and difficult questions posed by Londoners remained unanswered. Many users were left feeling that it was them, rather than the mayor, who had been taken for a ride.
"So @mayoroflondon chooses to answer questions about duck sized horses, but not about his 9 point plan for London," complained one follower. "So the #askboris session seemed to RT a whole bunch of questions and provide no answers...how very like a politician" complained another.
Boris later declared the session a resounding success with City Hall compiling an official mayoral report claiming that 553,076 users had been reached by the Twitter trend.
Whether any of those 500,000 were even remotely better informed about London issues is another matter, but in terms of promoting the Boris brand #askboris undoubtedly served its purpose.
Other politicians' Twitter question and answer sessions have not been quite as benign. Last year Ed Miliband was the target of particularly barbed questions on the site.
"If you give a speech, but nobody cares, do you make a sound?' asked one typically dismissive user on the site. "Do you feel bad about stabbing your brother in the back? asked many others.
What both examples show however is that so many politicians have completely failed to understand how social media works. For most users, conversations on Twitter and Facebook do not happen by prior appointment, but are part of their everyday lives.
Of course there are some politicians who understand this, and who actually use Twitter as a major part of their daily work.
But by setting up occasional brief Q+A sessions, Johnson and Miliband are almost asking for people to exploit and ridicule them. Not only are they sticking their heads in the public stocks, they are actually handing out sponges and cream pies for people to throw at them as well.
Of course being the Mayor of London or the leader of the opposition might not leave much time to spend answering endless questions on Twitter.
But if they haven't got the time to properly to get to grips with social media then perhaps they shouldn't bother trying at all.