Twitter users started it and other social services followed it. The hashtag originated in Twitter where the # symbol, called as hashtag, is used to mark keyword in a Tweet. A group of posts are grouped together by topic or type by use of hashtags. The Twitter hashtag search at the top of Twitter allows you search for a particular hashtag. The trending hashtags helps you to follow a particular topic on twitter-stream .Sometimes it will be like a running commentary of events and becomes really useful and interesting.
More and more social media services started supporting hashtags. That means you can search hashtags in comments or tweets in social media and micro blogging services to follow a particular topic. The twitter hashtags already proved its ability to create a group of engaged Twitterers. More recently Orkut added hashtag support. Google+ also provides improved hashtag support. When you use hashtagsin Google+ , they automatically link to Google+ search results.

Benefits of using hashtags in social services and websites
Hashtags provides a mechanism to explore all the related and grouped content in the service and also lets you to find more about the hashtagged topic quickly. If you Tweet or post with a hashtag on a public account, anyone who does a search for that hashtag may find your Tweet or post. This will give more visibility to your tweet. Similar effects will be there on all these social services. Since the grouping is based on the hashtag topic the probability of your post to appear in hashtag search is more. This without doubt will yield benefits to your online profile or brand.
Does Using hashtag affect the site’s SEO ?
SEO (search engine optimization) is all about the process of optimization to get effective indexing of your links in search engine index. As far as websites are concerned hashtags (#) refers to objects on the same page. So search engine usually ignores this and will not index those. Whatever after the # in the URL (for example http://www.example.com/#abcd) will be ignored. So hashtags effectively has no impact on SEO.
Benefits of referral links from #hashtags
Referral links from tweets that use hashtags usually beneficial to websites and blogs. Hashtags abuse is not very uncommon to extract these referral traffic juices. It is not a good idea to over-tag a single Tweet. Those over-tagged tweets will be considered as spam tags. This will defeat the benefits and purpose of #hashtags.
The Social Services and Websites that support hashtags
First Twitter started hashtag support and most recently Orkut supported it. Even non-micro blogging sites like YouTube supports hashtags in user comment systems. Usually in all these services all hashtags will be automatically highlighted in tweets, posts and comments and will link to the respective search page, where other posts with the same tag will show up. Here is the list of most prominent social services and websites that supports hashtags.
- YouTube
- Google+
- Orkut
- identi.ca
- Diaspora
Services like hashtags.org offers an overview of popular hashtags used on Twitter.
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