They're popular on Twitter as well as Instagram (although I'm yet to join that particular site so don't know this from personal experience). They are used on Facebook, but haven't really taken off as a marketing tool. And some data implies that they can actually decrease the reach of posts so I'd be cautious when using them there. They can be effective on Google Plus and are now an integral part of that site. In fact the network will automatically assign you a hashtag from the text you've included if you don't do so yourself. And you can find lots of related ones via the "Explore" function.
Still, when it comes to hashtag use and effectiveness, Twitter rules. Needless to say, you don't want to overdo them. Use no more than three per tweet. And give them a break now and then.
They can be employed in a variety of ways, including for local marketing. To lift the visibility of your Twitter account in this city for example -- and depending on the aim and subject matter of your tweets -- you can use ones like #Perth, #BeautifulPerth, #PerthyGreatness, #PerthBusiness, #PerthRealEstate and #PerthWeather. You can find others just by keeping an eye out for them. And you can even create your own, either for something generic or for a product, service or event you are promoting.
Because there are so many locals now on the site it's also worth using suburb level hashtags like #Subiaco, #Fremantle and #Joondalup. I know that I've been found via these specific ones and have garnered some new followers as a result.
And if you've got a blog then you can also put the odd local hashtag in titles, where appropriate. This is worth doing because if there are lots of people searching for a particular local hashtag on Twitter, then some are sure to be using Google to find it too. I did this with #foamroam in a previous post. It now ranks just under the website for the actual business it's promoting.
This tactic won't draw heaps of traffic. But the above example shows how easy it is to rank for these terms and it's sure to pay off over time with one or more such campaigns -- as long as the content you're posting is relevant and useful, of course.