Back in 2012, Google announced it’s intention to start using hashtags on the site. Originally, users would do this in a very similar way to Twitter, by inserting hashtags when writing a post to link similarly-themed posts together.
Last year, this feature was updated to make it more valuable.
Google Plus will now automatically generate the top three hashtags for a post. The algorithm for this is relatively complex, but the major point is what people are already talking about, and what will have the best SEO value.
These automatically-generated hashtags hold a lot more value than those generated by the poster. Google Plus hashtags will have a pre-populated page, containing relevant content, and this page is optimized to be easily found when searching Google.
For example, if you were posting something about painters, you might choose to hashtag it painters, painting, decorating, paint…If you leave this up to Google, it will decide what the top 3 hashtags are, and include those.
Not only will that then be linked to other posts about painting, painters, etc, but it’ll also show up on Google when someone searches for painters. That means more people will see the post, and be able to share or interact with it. Google does this often, because it’s a great way to show people that content that they want to read is on Google Plus, and therefore encourage new members!
If you write your own hashtags, Google will either not generate any automatic hashtags at all, or it will simply pick similar words to the hashtag that you chose, rather than using SEO/current search data.
Unless there is a good reason to use a specific hashtag, I’d recommend letting Google pick.