What does the # mean in twitter and facebook?

[Peter Post]

I am totally new to the whole social networking thing so can anyone please tell me what the # means in twitter and facebook?

[aye bee]

hash tag , they use it for trend or popular subject referring to the same subject or event.

For example:you just went to a concert for Eminem.

A lot of people also did.

So they will type

Wow the concert was awesome #eminem concert

others will say great party #eminem

[Andrea]

It's a hashtag, it's mainly meant for Twitter, but i guess people now got used to it on Facebook, EX:

I'm #Tired. You click Tired and you can see what a bunch of people put about #Tired.

[Joe]

It's a hash tag on twitter that turns blue and you can click it to see what other people are talking about that topic too. On Facebook it's a pound sign that people think they can use as a hash tag. Don't put a pound sign on Facebook. It's annoys the shit out of people including me