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How to Use hashtags when we are sharing content on facebook or other social network ?

Hashtags turn subject areas and key phrases into clickable links in you on your personal Timeline or Page. This kind of helps people find articlesabout subject areas they're interested in. To make a hashtag, write # the number sign along with a topic or term and add it to your post. For example:
I just saw the cutest puppy! #dogs
When ever you click a hashtag, you'll see a give food to of posts that include that hashtag. You may also see some related hashtags at the top of the page.
Make sure you keep in mind:
A hashtag must be written as a single term, without the spaces
You can include numbers in a hashtag, but punctuation and special characters like dollar and % won't work
You can search for a hashtag using the search bar at the top of any web page.
You'll only see articles that were shared with you
Find out more about who can see your posts if you use hashtags.

Hashtag is a word or phrase preceded by a hash sign (#), used on social media sites such as Twitter to identify messages on a specific topic.

People use the hashtag symbol (#) before a relevant keyword or phrase in their Tweet to categorize those Tweets and help them show more easily in Twitter search.
Clicking or tapping on a hashtagged word in any message shows you other that include that hashtag.
Hashtags can be included anywhere
Hashtagged words that become very popular are often

Using a hashtag in a social post is as simple as adding the '#' sign before a single word or phrase without spaces or punctuation (numbers are okay). 
Don't string too many words together with a single hashtag.
If you tweet with a hashtag on a public account, anyone who does a search for that hashtag may find your tweet.
Don't #spam #with #hashtags. Don't over-tag a single tweet.
Use hashtags only on tweets relevant to the topic.

How to Use Hashtags (DO!)
Do be specific when using hashtags
Try and hone in on a passionate community that shares an interest in one specific theme. The more specific you can get with your hashtag, the more targeted your audience will be—and a targeted audience generally means better engagement. If you don't have your own business hashtag, find one or two existing ones that really fit the photo. To use an example from Instagram's own tips for using hashtags, if you're after Volkswagen lovers the hashtag #vwvan will earn you a lot more of your desired followers than #van will. Twitter has created a helpful infographic to help you choose the right hashtag.
Do cater hashtags to the social network you're using
While hashtags on all social networks have the same fundamental purpose of content tagging and discovery, the use of hashtags still varies by network. In terms of use, Instagram hashtags, for example, are often more focused on description of the photo and the tools used to take it than on a broader story or theme. This is at odds with Twitter, where hashtags tend to be more focused a topic of conversation, or a group of people (a chat for example) that you would like to engage.
Before using hashtags, do research on the proper way to use them for that particular network. Most networks will have guides for hashtag selection and use (here is Twitter's). Also take the time to discover the most popular and most relevant hashtags on a specific subject for each network. This extra time you invest will pay off in engagement down the road.