Twitter hashtags

Hashtags are used on Twitter to mark a topic, so that people can follow a group of tweets on the same topic by various people.  So, for instance, this one is to make the tweet stand out to people who are searching for tweets about language:

[video & article] Children developed own #language in cave http://bit.ly/bnKJDL 

But people are using them for other things, where it's clear that it's not to identify a topic or theme.  Here are a few examples: 

You know when you've worked really hard revising a story, then suddenly realize you were revising an old version? #pullhairoutofhead #cry

 Me: "I'm not accepting new work now." Would-be client (self-pub author): "Well, can you just review what I've done myself?" #no

 ugh, the intros are the hardest part. Spent three hours on 3pp writing. Still not sure if it's too-detailed or not-enough. #onward

 SO many ways to get hold of a book these days. I mean there's really no excuse http://bit.ly/myfryapp #shamelesswhoring

So, why and how do people use hashtags?  What does it serve to put it after a # rather than just saying it? 

Down and Up in the UK

A former English Language student (now a Doctor of Psycholinguistics--yippee!) and I were talking about doing a study of how people use and understand 'down' and 'up' in directions in British English.  There are some issues with London, Oxford, and Cambridge having 'up' status for various people in various contexts, but there's also the question of whether 'down' is perceived as 'south' or if it's 'toward the coast' or if it shifts depending on whether you're on a hill or not. 

There's been a lot interest in prepositions in Cognitive Linguistics, but I don't think this aspect of down/up has been examined.  Part of the question is: do English speakers understand down/up to be 'cardinal directions' (i.e. like north/south).  Something has been made of the fact that English speakers give directions with left/right, rather than north/south/east/west.  Does use of down indicate that we are more aware of the cardinal directions than might be thought from the relative lack of cardinal direction terms in our language use?