LONDON (Reuters) - Facebook, the social network giant, is set to begin showing advertisements to users on mobile devices before its $5 billion initial public offering, the Financial Times reported on Monday.

Facebook has already discussed proposals with unidentified advertising agencies for displaying what it calls "featured stories" in the news feed seen by users on the site in an effort to tap a new source of revenue, the newspaper cited several people familiar with the plans as saying.

Facebook mobile device users are likely to see advertisements in March ahead of its initial public offering, which is expected in May, according to the FT.

(Reporting by Stephen Mangan; Editing by Paul Tait)