Here's a ranked listing of Fonts.com Web Fonts' top 100 most used Web fonts for June 2013:

Neue Helvetica

Proxima Nova

Trade Gothic

Futura

Gill Sans

Helvetica

Univers

DIN Next

Avenir

Avenir Next

Neue Frutiger

Frutiger

Linotype Univers

Frutiger Next

ITC Avant Garde Gothic

Trade Gothic Next

Humanist 777

Century Gothic

Futura T

Museo Sans

Univers Next

News Gothic

Adobe Caslon

Neue Helvetica

Neo Sans

Optima

Linotype Didot

Avenir Next

Baskerville Classico

Bembo

Arial

Agilita

Rockwell

Monotype News Gothic

DIN 1451

PMN Caecilia

ITC Franklin Gothic

VAG Rounded

Bodoni LT

Soho

Myriad

Brandon Grotesque

ITC Garamond

ITC Lubalin Graph

Corporate S

Helvetica Monospaced

ITC Conduit

News Gothic No.2

New Century Schoolbook

Twentieth Century

Soho Gothic

Neue Haas Grotesk

Sabon

Abadi

Gill Sans Infant

Adelle

Trade Gothic Next Soft Rounded

ITC Officina Sans

Times

Bembo Infant MT

ITC Century

Palatino

Neue Helvetica eText

Sackers Gothic

Klint

Frutiger Serif

Eurostile LT

Letter Gothic

Calibri

Neue Helvetica Arabic

Laurentian

Eurostile Next

Harmonia Sans

Bauer Bodoni

Camphor

Helvetica World

Garamond 3

Akko

Museo Slab

Memo

Adobe Garamond

ITC Franklin

ITC Fenice

Clarendon

Yakout

Effra

Minion

Novecento

MSung

New Caledonia

Monotype Baskerville

Plantin

Sassoon Sans

Museo

Ascender Serif

Compacta

Slate

Cachet

ITC Officina Serif

Aachen