This great little article from Buffer gives you everything you need to know about using hashtags on your Twitter.

The science of using hashtags on Twitter.

In summary:

• Tweets with hashtags are likely to get twice as much engagement than without
• Don’t use any more than two per tweet! Any more than two leads to LESS interaction.

Also, don’t use hashtags on Facebook, posts perform better without them!

Check out the full Buffer article here.