The meaning of ‘a popular Twitter phenomenon’

The meaning of ‘a popular Twitter phenomenon’

The passage below is from The Library: A Fragile History by Andrew Pettegree.

Bookmobiles were phased out entirely in Texas in the 1970s. But there were still over a thousand operating in the United States in 2015, and 650 in 2018.35 This remains a popular form of library service in many parts of the developing world: in India and South East Asia, Kenya and Trinidad, as well as north European countries like Norway and Finland with large rural hinterlands.36 By the last

third of the twentieth century, the mobile library had become a very visible symbol of the globalisation of library culture. The Biblioburro donkey library of Colombia, the iRead mobile library of Nigeria and BiebBussen (converted shipping containers) are only the most colorful examples of the ingenuity of the library profession in bringing books and information services to widely dispersed and economically disadvantaged communities. Whether they will survive the shift to worldwide digital access is an open question; for the moment, readers continue to welcome their regular visits, not least those of the BookyMcBookFace library on the islands of Orkney, also a popular Twitter phenomenon with 70,000 followers.

On this paragraph I was curious about the meaning of ‘a popular Twitter phenomenon’. When I googled, the BookyMcBookFace library had a twitter account and it’s number of followers were about 70,000. Does the underlined ‘a popular Twitter phenomenon’ refer to ‘the BookyMcBookFace library’? (Am I right?)

One more question. Then why did the author use ‘a’ instead of ‘the’? In my sense of the English grammar, if we know the identity of a noun we use ‘the’ not ‘a’.

Thanks in advance.