Cost information: Includes multiple tiers. Free tier allows for 3 social media accounts and 10 queued posts. $15 tier allows 8 social media accounts, 100 queued posts, a scheduling calendar for upcoming posts, and basic analytics of post performance (this level should be enough for almost any library). There are more expensive levels with advanced analytics, provisions for multiple team members, and up to 2000 scheduled posts.
How it works: Buffer is a tool for coordinating posts across multiple social media accounts with a single sign-in, tracking their performance, and targeting posts toward specific audiences (a fun picture might do well on Instagram or Pinterest, where a text-heavy post may be better suited for Twitter or Facebook, for instance). It allows you to schedule multiple posts at a time, and has apps for web, iOS, and Android, all of which let you use the scheduling calendar at the $15 "pro" tier.
How could it be useful in a library: Social media can be a cheap way to provide marketing for library happenings, and is practically expected from any public entity. However, it can be time consuming to juggle multiple accounts, especially considering that few libraries have the budget to allow for a full-time marketing person, let alone a social media marketing position. For relatively low cost per month, the single Buffer sign-in can be distributed across trusted individuals in the library, who can then schedule posts as time allows, ensuring that they'll be sent to every platform where the library has a stake. The analytics can also provide justification for time/money spent, showing hard data on which posts are well-liked, and acting as direct patron feedback--"We like it when you host events like this" or "We don't particularly care about this information" can be equally valuable to guide future programming.
Types of training required: Minimal social media best practices are ideal, and an understanding of how each individual platform works will allow the library to avoid gaffes like trying to put links in an Instagram post or putting text-heavy content on Pinterest. Otherwise, the app and web platform seem simple and intuitive, even offering suggestions for ideal times to post on each platform. Knowing when and how to post is almost as important as the content, so the fact that Buffer removes these obstacles is a major plus.