Here's a listing of the top 100 most used fonts from the Fonts.com Web Fonts service for December 2013:

Neue Helvetica

Avenir Next

Trade Gothic

Proxima Nova

Univers

Avenir

Gill Sans

Futura

Frutiger

ITC Franklin Gothic

Helvetica

Linotype Univers

Sabon Next

Klint

Chaparral

DIN Next

Century Gothic

Arial

VAG Rounded

Museo Sans

Gill Sans Infant

ITC Caslon No. 224

Eurostile LT

Bree

Myriad

Neo Sans

Univers Next

Rockwell

ITC Avant Garde Gothic

Museo Slab

Neue Helvetica eText

ITC Legacy Serif

Praxis

ITC Century

ITC Lubalin Graph

Diverda Serif

Motoya Birch

Amasis

Optima

Clarendon

Neue Frutiger

ITC Legacy Square Serif

Swiss 721

Amadeo

ITC Bodoni Six

Trade Gothic Next

Helvetica World

Brandon Grotesque

ITC Officina Serif

Azbuka

Soho Gothic

Chocolate

ITC Conduit

Frutiger Next

ITC Charter

Comic Strip

Linotype Didot

Lexia

ITC Officina Sans

Calibri

VAG Rundschrift

PMN Caecilia

Zapf Humanist 601

Alternate Gothic

ITC Fenice

Trade Gothic Next Soft Rounded

Bodoni LT

Slate

Delima

Monotype News Gothic

Adobe Garamond

Orator

Rotis Sans Serif

ITC American Typewriter

Bookman Old Style

Soho

Neue Helvetica Arabic

Humanist 777

Droid Sans Mono

ITC American Typewriter Hellenic

Swift

ITC Eras

Linotype Sketch

Bickham Script

Funkydori

Birch

Caliban

Rosarian

ITC Stone Informal

Perpetua

Auriol

Egyptienne F

Bembo

Sackers Gothic

Miss Donna

Paris

Greyton Script

Sugar Pie

Caslon Classico

Droid Serif