Its a great idea, you want to get a bit of positive word of mouth. You start an online campaign and so do a bit of crowdsourcing. You find an appropriate hashtag (the symbol which placed before a term allows searching and grouping of tweets) and invite Twitter users to say what 'makes them smile'. In theory it creates a buzz about the brand and gets coverage online and offline. More importantly, in theory, it associates the brand with the making people smile. Well this was what could have happened for Vodaphone in a parallel universe when they offered free phones to those who told the world what made them smile.
The major mistake was to not take into account the environment for the brand. If you are a brand ticking along just searching for a bit of positive coverage it can work fine. Tetley, the tea company, invite users to send in pictures of their mugs to 'The Gaffer' at @tetley_teafolk; it seems to be going ok but Tetley don't have enemies. Vodaphone face significant criticism for not paying taxes in the UK. UK Uncut, campaigning against cuts and tax dodging, invited their followers to hijack #makemesmile. Very soon it became a trending topic, the content said it all. See this article for examples.
Just like Labour's #changewesee, Vodaphone managed to attract opponents which shaped the messages at key times. Conservative/LibDem supporters placed a lot of observations of negative changes they saw on Labour's site. The lesson is that letting the online crowd have control over your message is a dangerous one, but the outcomes can be predictable. Basically if your brand is in trouble, possibly avoid giving your critics a space on your own website to say what they think. It can end up with your opponents getting the positive coverage and not you!