London vs Dublin, which is the best city to run an start-up on social media and start to raise money?

Dublin. Lower costs; talented, flexible, young workforce; part of the Eurozone; 10 of the world's largest ICT/digital players creating a formidable tech cluster; high availability of seed funding; attractive corporate tax rates; and the support of one of the world's leading innovation development agencies, Enterprise Ireland. Full disclosure: I work for the Irish government to attract high-growth startups to Ireland, more details on our programme at http://startinireland.com :)

Unless you are going through Irelands goverment for funding. Then London is the place to be. London may be more expensive but that is because it is more popular. Which means more talent and more investors. Transport is second to none and the infrastructure is first class.