what does the # sign mean when using it on facebook and twitter?

[pluto]

[Ben]

On twitter, the so-called Hash Tag is used to signal a trending topic. You'll put it in your tweet (which many people then mirror to their Facebook status) so that you'll get counted among the people commenting on something in particular. By using the # sign to signal the topic, you make sure that you don't get any random comments when you search for it.

[Stephy Rox!!!!]

well in twitter it is used so when you click over that u can see more results that have a tweet with the same word.
Ex.
#Pretty

click

tweets:
{Taylor is so #pretty}
{My new dress is so #pretty!}
{I look totally not #pretty today D: }
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hope it helped