Which is the "most serious"/payable business, allowing users to login with FB?

[gyzar]

I can only think of Skype and Groupon. Would you mind helping me with a few more examples?<p>Muchas gracias.

[opionion-matter]

Spotify is one I can think of.

[Irishsteve]

Ticketmaster

[gyzar]

I can't find the FB login on Ticketmaster ... Where is it? Thx.