Why do the use the hashtag #fb on twitter?

[David]

i know it means #facebook ... but i dont get the reason behind adding that tag to a quote or joke or oneliner or anything... why would ppl do that?

[pseudonym]

If you have linked your twitter account to your facebook account, but you only want some of your tweets to appear on facebook, you put the hashtag #fb in the tweets you'd like sent to your facebook account. The tweets without this hashtag will be ignored and remain only on twitter.