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

Neue Helvetica

Trade Gothic

Gill Sans

Helvetica

Avenir

Futura

Univers

Proxima Nova

Avenir Next

DIN Next

ITC Avant Garde Gothic

VAG Rounded

Swiss 721

Neue Frutiger

Frutiger

Frutiger Next

Linotype Univers

Trade Gothic Next

Humanist 777

Adobe Garamond

Soho

Adobe Caslon

Arial

ITC Conduit

Century Gothic

ITC Lubalin Graph

Museo Sans

Baskerville Classico

ITC Franklin Gothic

Rockwell

Univers Next

Neo Sans

Optima

Bembo

Linotype Didot

Agilita

Myriad

PMN Caecilia

Linotype Feltpen

Bodoni LT

DIN 1451

Helvetica Monospaced

Heisei Kaku Gothic

Joanna

Brandon Grotesque

Neue Haas Grotesk

ITC Garamond

Monotype News Gothic

Memo

News Gothic

News Gothic No.2

ITC Century

Gill Sans Infant

Sabon

Adelle

Abadi

Twentieth Century

Soho Gothic

New Century Schoolbook

Harmonia Sans

Trade Gothic Next Soft Rounded

Bauer Bodoni

Aachen

Corporate S

Neue Helvetica eText

Letter Gothic

Bembo Infant MT

Eurostile LT

ITC Officina Sans

Times

Frutiger Serif

Sackers Gothic

Slate

Laurentian

Eurostile Next

Akko

Neue Helvetica Arabic

Calibri

Novecento

Palatino

Museo Slab

Klint

Camphor

Minion

Helvetica World

Clarendon

Sassoon Sans

New Caledonia

ITC Fenice

Futura T

Strayhorn

Museo

Monotype Baskerville

Garamond 3

Glypha

Basic Commercial

Yakout

Smart Sans

Alternate Gothic

Plantin