What is it?
Instagram is similar to Pinterest it many ways. It is also a visual-only social network, where users share only images. However, while on Pinterest the images are reblogged from other websites (or in some cases uploaded from the user's computer) on Instagram they are photographs taken directly with the user's smartphone (or an mobile device).
Moreover, Instagram is an application, and its web counterpart is simply an addition - while Pinterest is web-centric, and the mobile app is a mirror of the website, with a user experience optimized for touch and smaller screens. The Instagram app uses the device's camera to take photos, crops them to a square and allows the user to add filters and decorations. The photos are then shared on the Instagram network, with an option for Facebook and Twitter sharing as well. Short captions and hashtags can be added to the images when they are shared.
Instagram photographs imitate the look and feel of film and Polaroid. Users can add a white Polaroid frame, textures, blur and color effects to give an aged vintage look the image.
Users can also record videos lasting for a maximum of 15 seconds.
How is it used?
Similar to Facebook and Twitter, Instagram is widely used to share snippets of everyday life - in this case, using only photographs (with short captions and hashtags).
The built-in filter functionality is widely embraced by users, and considered to make even mundane images look artistic. The photographs shared on Instagram usually capture daily objects and actions and resemble visual tweets. Images of food are very popular, for example.
Other popular topics include "selfies," fashion and make-up, vacations, animals, signs and texts.
Hashtags are extremely popular. There's a limit of 30 hashtags per image. For instance, a widely spread one is #nofilter, which signifies that the photo is original, with no filters added.

Businesses have quickly adopted using Instagram as well.
Unlike Pinterest, selling products is not as successful here. Instagram is used in a more personal way and users are trying to get a peek into other's lives (like Facebook and Twitter) rather than looking for products.

Celebrities are also using Instagram to connect to their fans in a more personal way, sharing snippets of their daily lives as well as big events (music and film awards, premieres, etc.)
Finally, Instagram is starting to become popular with young artists and designers, who use it to share the progress of their works, final pieces, or close-up details.
Audience
Like Pinterest, the majority of Instagram's audience is female - over 68%.
Over 90% of the users are under 35 years old, which makes the network great for fashion and apparel brands, entertainment and media companies.
Marketing
Instagram marketing would be a great solution for:
- food & beverage
- fashion / apparel brands
- music & entertainment industry
- hispitality
- individual bloggers
- artists and designers
SEO
Just as Pinterest, Instagram links are nofollow and do not count for SEO. The images are static and cannot be linked to external pages either.
Thus, Instgram has little to none value for SEO.
User Engagement
This is where Instagram's strenght lies. It is a perfect platform for running contests, telling stories and engaging users with the brand.
A widely spread tactic is to have users upload photos with a specific hashtag (usually including the brand name) and award one of those photos (with a gift card, discount, etc.)
Many brands have created their own hashtags that do not include the brand name, for instance Red Bull - #GivesYouWings. They give users incentives to use these hashtags, raising brand awareness an visibility.
Free People, a US fashion brand, are an excellent example of optimal Instagram usage. The brand uses it in a way a regular user would, sharing daily photos of the employees' lunches, vacations, and moments in the office. Apart from this, they run regular contests that require users to use their hashtags.
For their latest Instagram campaign, Free People are asking users to upload self-portrait photos, shot in a specific frame: a full-body of the user, holding flowers. The submitted photos are going to be used later in a stop-motion video for Earth Day, and the best one will be awarded with a voucher. The brand gets to spread their hashtags and the users get to see themselves in the popular video. Also, the brand values (environmental awareness, appreciation for nature) are reinforced.