Important: Twitter releases a timeline widget to rival Facebook Like Box.

Twitter launched a timeline widget, which you can embed on your website. It shows latest Tweets from a given account, tag, search term etc... You can instantly Tweet, Follow, Retweet or Reply from that widget.
This is a perfect alternative to Facebook's Like Box. Why? Because since the EdgeRank algorithm was introduced, (by some study) only about 16% of fans of a page on FB get to see posts made by that page. It doesn't really matter for me, if it's 16% or 80%. What matters is this: when someone follows my brand in any way, he should receive all my updates and it's up to me to make them worthy. Facebook doesn't provide that (unless you pay about $5 to deliver a Promoded Post to about 1000 fans), so I will be switching all news feeds from http://essentialsoftheonlinebusiness.com to this Twitter widget. Also, if I have a longer post (like this one), I will Tweet the G+ link for it rather than the FB one.

As for Google+ - you can use some 3rd party widgets like 
http://gplusapi.appspot.com/ but they rely on the Google+ API which lets you get data from Google+ 10.000 times a day. Unless it's cached properly (which I'm pretty sure this widget is not doing), it will break on sites with over 10k pageviews / day.

Last imporant thing: your site may have a lot of visitors from a particular social network. You can check if it's worth it to switch from FB Like Box to the Twitter widget using this WordPress plugin http://adaptpartners.com/internet-marketing-tools/social-media-user-detection/ (you don't have to download it from that page manually, you can find it in the WP repository) or this tutorial for any other website: http://www.seomoz.org/blog/visitor-social-network-login-status-google-analytics

Here's the news about the Twitter timeline widget with examples:
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