Official comment
In June, 2016, Brendan Dubbles, our Postmaster at the time, also provided this information:
The promotions tab in Gmail is a tricky thing. There are many things that govern what folder messages are sent to in Gmail. The largest of those is user interaction. If people move your messages into the promotions tab, Gmail will filter your messages as such.
Another large factor is the number of links you use in your emails. If you have a over 3 links to promotional sites along with social links, the message looks promotional.
A good way to think of it while constructing your emails is that you are sending an email to your grandmother. If you were to send a personal email to your gran telling her to checkout a cool new site you found, you would not include 3 links to the same page as well as links to your Facebook, Linked-In, and Twitter accounts.
A good rule of thumb is to focus your messages around a single call to action and a conversational message rather than several links to various promotional pages and your social media.
Another thing senders can do to improve inbox placement with Gmail is to put a message in your email asking that people move your messages into the inbox to be sure that the recipient sees them. Gmail will track that user interaction and take it into account when placing your messages. If you would like someone to inspect your messages and provide some coaching for you, get in touch with my team at "postmaster@ontraport.com".