Facebook

Advertising Post image – 1,200 x 628px (Ratio 1.9:1)
Minimal text is best but not a restriction – view more

Google Plus

Post image – 750 x 525px (Ratio 1.42:1)

Instagram

Advertising Link Post image – 1,080 x 1,080px (Ratio 1:1)
Minimal text is best but not a restriction – view more

Linked In

Sponsored Content image – 1,200 x 627px (Ratio 1.91:1)
Display Adverts – 728×90, 300×250, 160×600, 300×600

Twitter

Single Tweets – 1200 X 675 (generic size to cover desktop & mobile) but ratio is between 1:1 – 2:1
Single app or site promo – 800 x 320 pixels
Lead gen – 800 x 200 pixels

There are more details options on their advertising pages
Facebook
Instagram
Linked-In
Twitter