Google Plus has just implemented Hashtag same as twitter for better and real time search result. This function as similar to twitter hashtag # function, i.e putting # before main keywords while sharing it and if you want to search any thing by hashtag then go to search box type # hashtag before your search or keyword and you will get the real time latest result. Even if the shared post doesn't have the tag still you will get the best real time result.
Guide to use Hashtag in Google Plus
Below two images will give you full idea about how to use hashtag in google plus and how to use google plus search.
Putting # before your keyword will make it searchable for Google plus easily
The post shared with # hashtag always ranks above other search
Till now there is no limit of using hashtag in a single sharing but don't exceed it more than 3 tags per sharing, because Google may anytime such rules.
Guide to use Hashtag in Google Plus
Below two images will give you full idea about how to use hashtag in google plus and how to use google plus search.
Putting # before your keyword will make it searchable for Google plus easily
The post shared with # hashtag always ranks above other search