The most popular of all social networks. Use this to promote your library and provide links to the library website.
A micro-blogging site that is very popular with younger people. Great for announcements, promoting events and connecting with other organisations.
Pinterest is not used nearly as much as other social networking sites in the library realm, however this is beginning to change. It allows libraries to communicate visually but really needs to be promoted by a Twitter or Facebook account to be effective.
There are various ways of starting a blog. These a great if you have a niche audience who like to know the details about some of the goings on at your library. Guest blog posts are also a great way to attract new audiences as blog followers are some of the most loyal social media followers around.
As videos become easier for people to make and share, video hosting sites are becoming more and more important. They are particularly useful for some of the larger libraries and institutions that have the time and money to produce their own short videos.
Scoop.it allows you to 'curate' articles and resources and is probably more likely to be used by a specific and existing library community rather than the general public.