As Twitter is invaded by the masses and loses its cool, experienced users are parading their know-how by littering tweets with increasingly arcane codes: RT for re-tweet of course, L: for location and # for hash-tagging a post with a topical or popular word – #iphone or #lost for example.

Some of this is the simple fun of working within short sentences – the Twitter equivalent of OMG! txt spk – but I think hashtags mean something more.

Hashtags allow people to join a virtual club easily and temporarily. And to leave it just as quickly. People can just dive into a conversation, make a pithy observation about the #brits and dive right out again. No need to subscribe to an email list, no forms to fill in, no facebook group to join. It can be a no-complications, one-tweet stand.

They are perfectly suited for an world where attention is ever-more scarce and people are wary of sharing personal data. But they’re also wonderful fun – the ideas flitting in and out of existence in perfect harmony with their true value and popularity.

They’re hashmemes if you like.

Right now #oscars is in vogue but once the red carpet is rolled back up, it’ll disappear as a living idea until the next time enough of us shine a torch on it.

Hashtags.org allows anyone to see what’s hot at any moment, and even includes graphs describing most hashmemes’ beautifully short lifespans.

Marketers have to work pretty hard to jump on such fast-moving bandwagons. Indeed the only way to do it is to keep running. Only if you’re already up to speed with the conversation can you expect your brand to contribute something useful and credible.