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

Neue Helvetica

Avenir Next

Gill Sans

Trade Gothic

Avenir

Univers

Proxima Nova

Gill Sans Infant

Rockwell

Futura

Helvetica

Classical Garamond

Linotype Univers

Frutiger

Klint

DIN Next

Courier PS

Century Gothic

Museo Sans

Myriad

Bree

Slate

ITC Franklin Gothic

ITC Avant Garde Gothic

Arial

Museo Slab

Harmonia Sans

VAG Rounded

Chaparral

Univers Next

ITC Legacy Serif

ITC Caslon No. 224

Eurostile LT

ITC Century

ITC Lubalin Graph

Amasis

Motoya Birch

Neo Sans

Neue Helvetica eText

Optima

Neue Frutiger

ITC Legacy Square Serif

Trade Gothic Next

Swiss 721

Delima

ITC Officina Serif

Lexia

ITC Charter

Calibri

ITC Officina Sans

Soho Gothic

Droid Sans Mono

Bodoni LT

ITC Conduit

Frutiger Next

Bookman Old Style

Linotype Didot

ITC American Typewriter

PMN Caecilia

Adobe Garamond

Auriol

Swift

Helvetica World

Soho

ITC American Typewriter Hellenic

Neue Helvetica Arabic

Egyptienne F

ITC Stone Informal

Linotype Sketch

Glypha

Monotype News Gothic

Francker

Rotis Sans Serif

Bembo

ITC Fenice

Monotype Goudy

Droid Serif

Azbuka

Comic Strip

Humanist 777

ITC Eras

Bodoni

Sackers Gothic

Copperplate Gothic

C Hei 2 PRC

Rileyson

Caslon Classico

M Elle PRC

C Hei PRC

M Lady PRC

Adobe Caslon

M Stiff Hei PRC

Twentieth Century

Orator

VAG Rundschrift

Droid Sans

Andale Mono

Copperplate

Inform

Clarendon