I realize this question is old, but it's still a top google hit, so I thought I would give an updated answer that includes popular social media sites.
I generally have four groups of meta info:
Regular Meta Info - used by search engines and browsers
<title>{{pageTitle}}</title>
<meta charset="utf-8"><!-- html5 version of http-equiv="Content-Type"... -->
<meta name="description" content="{{description}}">
<meta name="keywords" content="{{keywords}}">
<link rel="author" href="https://plus.google.com/{{googlePlusId}}" />
<link rel="canonical" href="{{pageUrl}}" />
Facebook Meta Info - used by Facebook when someone shares your url
<meta property="og:url" content="{{pageUrl}}">
<meta property="og:image" content="{{imageUrl}}">
<meta property="og:description" content="{{description}}">
<meta property="og:title" content="{{pageTitle}}">
<meta property="og:site_name" content="{{siteTitle}}">
<meta property="og:see_also" content="{{homepageUrl}}">
Google+ Meta Info - used by Google+ when someone shares your url
<meta itemprop="name" content="{{pageTitle}}">
<meta itemprop="description" content="{{description}}">
<meta itemprop="image" content="{{imageUrl}}">
Note: you don't really need these, Google+ will fall back to the Open Graph tags that Facebook uses.
Twitter Meta Info - used by Twitter when someone shares your url
<meta name="twitter:card" content="summary">
<meta name="twitter:url" content="{{pageUrl}}">
<meta name="twitter:title" content="{{pageTitle}}">
<meta name="twitter:description" content="{{description}}">
<meta name="twitter:image" content="{{imageUrl}}">