People using "#" on facebook/twitter?

[alexbmwz8]

This may sound like a stupid question, but I see people writing "#Blahblahblah" online all the time and I'm not sure what the "#" is supposed to do..if you can explain then please do, thanks

[Gabbie❤]

The # symbol, called a hashtag, is used to mark keywords or topics in a Tweet. It was created organically by Twitter users as a way to categorize messages.

People use the hashtag symbol # before relevant keywords in their Tweet to categorize those Tweets to show more easily in Twitter Search.
Clicking on a hashtagged word in any message shows you all other Tweets in that category.
Hashtags can occur anywhere in the Tweet.
Hashtagged words that become very popular are often Trending Topics.

Doesn't work on Facebook, people just like hashtags.

Example: @VegNews added the hashtag before the word "vegan" in their message. The word is now a link to search results for all Tweets containing "#vegan" in the message.

[Nessie Weber]

On Twitter, it's called a hashtag. It's an easy way to get to other people's tweets who share something in common with you, and people use it to talk to a lot of people at once. It doesn't have nothing to do with Facebook, and I don't understand why people put them on Facebook, because they don't work or belong there.

[Tim]

It's their thought/opinion after stating something.

Examples:

Going to the mall tonight.#pumped

About to ask this really cute girl out.#nervous

[ScσττRΛSC³]

On Twitter thats what begins what is known as a hashtag. Its sorta a keyword for people to find people commenting on a particular topic quickly. For example when the G4 show Attack of the Show is on they encourage their viewers to tweet during the show & to use the hashtag #aots.
As far as it being on Facebook, its probably that people mistakenly assume that it has the same effect on Facebook as it does on Twitter. It doesn't. Much like when people use (8) or (F) both of which are commands used in Windows Live Messenger for emoticons. Again people mistakenly assume that they would have the same effect on Facebook when it doesn't.