Services that work well on cellphones:
Google > images (6k bytes per image)
Services that work just OK on cellphones:
twitter (10k bytes per update, and most features not available, and no way to log out hence no way to log into different account on same phone; Update 2012.Feb.14: no longer works at all on cellphone, gives error "Insufficient memory for this operation"; Update 2015.Oct: for a year or so twitter.com and mobile.twitter.com each fail with "Security Error: Untrusted server certificate")
Services only for desktops/laptops: