How do you use hashtags (#) on Twitter and Facebook?

[Cemiad]

So I know by adding the hashtag before a phrase it adds your tweet to a list of tweets who were also tagged with the same word. This makes sense when using phrases or words like #obama or #(insert band name here), but I don't understand why people would tag something like #donttrustmewiththatknife #ever for a description of a Facebook picture. Two people commented on the #donttrustmewiththatknife #ever picture with, 1) #smallslowcuts and 2) #millionlittleones. First of all, who would ever search for #smallslowcuts in a tag, and secondly, who would search tags in the first place? Why use the hashtag symbol on a picture on Facebook when it doesn't connect to Twitter (FB status updates and Twitter are connected though, right?)? And finally, I really don't understand why people use Twitter. Do people go throughout their day saying things like, "Bum just made scene on the bus #hilarious". I just don't have any desire to know little things like that nor do I have the desire to tell things like that. What else do regular people use Twitter for?
Thanks

[Lozza]

I personally can't stand Twitter, but it's obviously very popular. I guess the whole point of hashtag is that you can hashtag anything. The weirder the better, I guess. Cos then you're not "following the masses" as it were, but creating your own list. A bit of freedom in the robotic world of social networking.

[tish]

Hashtags on Twitter are mainly just for fun and to help you get your point across in 140 characters or less, but hashtags are completely useless on Facebook. And Facebook and Twitter statuses are only connected if you want them to be (by going to your Twitter settings and clicking on "post your tweets to Facebook").

[Chris]

OK, here you go... hashtags 101 :

Hashtags are used to help you search for and keep track of things (conversations, trending topics, etc..) on Twitter. For example, I tweet about job search. So in a typical tweet, I may say something like;
#jobseekers - Check out my #blog for helpful information re: #resume and #jobsearch tips.

So, if a person did a search on any of those hashtaged words, my tweet would show up in their search.

Get it?
There are some hashtags that are ALWAYS used, for example, #followfriday, #jobs, #obama, #newyorktimes.... etc....
Others are temporary and may be used if a particular story is trending in the news, for example, #spaceshuttle or perhaps a popular celebrity #justinbieber - really, the ideas are endless.
Some people try to START a trend by using a hashtag in front of .... well... whatever they want to, really. At this writing #TheWayISeeIt is trending in New York. Someone started it for whatever reason and others saw it and jumped on board.

There are many sources for more information on hashtags. Here are a few that you can check out. I didn't get it at first, but now that I use Twitter for a REASON, I think hashtags are great. They can really help you to narrow down your search.