* Main Entry: 2media
* Function: noun
* Inflected Form(s): plural me·di·as
* Usage: often attributive
* Etymology: plural of medium
* Date: 1923
1 : a medium of cultivation, conveyance, or expression; especially : medium 2b
2 a singular or plural in construction : mass media b plural : members of the mass media
usage The singular media and its plural medias seem to have originated in the field of advertising over 70 years ago; they are still so used without stigma in that specialized field. In most other applications media is used as a plural of medium. The popularity of the word in references to the agencies of mass communication is leading to the formation of a mass noun, construed as a singular . This use is not as well established as the mass-noun use of data and is likely to incur criticism especially in writing.
under "medium": definition 2 bplural usually media
(1) : a channel or system of communication, information, or entertainment — compare mass medium (2) : a publication or broadcast that carries advertising (3) : a mode of artistic expression or communication (4) : something (as a magnetic disk) on which information may be stored
Conclusion: "the media are powerful because they influence public opinion" is correct. "the media is powerful because it influences public opinion" is increasingly accepted and will no doubt replace the plural construction at some point.