if all the world's tweets bearing the tag #family were gathered together ... well, there would be an awful lot of them and it would be no use at all as an aid to finding the ones you want. But if you were to use something like #loftylottiesfamily it might help you and your kin to locate them.
they're very common on Twitter, not so much use on lesser social media like AB... but if you want to issue a witty, trenchant tweet on some matter of national importance, and you attach a trending hashtag to it, the newspapers might pick it up for the news reports on their own websites and you'll be famous.
Pat, they don't work here but if you use it on a search engine it will bring up a - say, there was a thread here the other day about a £10 christmas bonus, go on your search engine and do the # as #£christmasbonusuk and you will open a right load of conversations and info on it, I don't consider AB as Social media but on TikTok, Instagram and Facebook, # has it's uses