X (previously Twitter) - Do you know when to use hashtags vs mentions?

So if it is about the topic, then the hashtag is the way to go:

Microsoft releases #VisualStudio 2013 Update 2

On the other hand, @Mentions are used when talking to or about someone (the user account of a person, brand, group, etc.). The link created by a mention goes to that user account. The user is notified when they have received a mention in a post so that they can respond.

So if you are talking to someone directly, or mention a specific user in your tweet, you should use the @mention:

Thanks to @visualstudio for the map to Bill Gates’s house – I’m looting his safe right now!

The best tweets will use a combination of @mentions and #hashtags where appropriate to properly leverage the platform – that way readers can check out the people involved OR join the conversation about a topic.

Figure: This tweet utilizes @mentions, #hashtags, and a link. These will create higher engagement and let the reader further explore the topic

• Small companies: If the brand is not big you don't need it diluted. So find the profile and use the @ not the #. E.g. @firebootcamp is preferred over #firebootcamp
• Large companies: If the brand is big, you will see a difference between the @ and the #. So if you want to tweet about a subject that is also a username – such as #VisualStudio / @VisualStudio - then the content of your tweet will decide whether to use a hashtag or a @mention: