There are many plugins which allow you to share your posts to social networks easily. But among those which I’ve tried I like this one. (In WordPress dashbord it is called Twitter Facebook Social Share and is made by Kunal Chichkar). You can see it my web-site, it is small line under the title and for me it’s just the way I wanted it to look.
There are dozens of social networks. But if you are going to make accounts everywhere and share your posts everywhere you will loose a lot of time. In this case you’d better use ping.fm. And the plugin I’m talking about is not for you but for the visitors of your web-site. And I think it includes 4 main networks where people hang. Twitter is by many people and it’s only 1 click to make retweet if someone likes your post. Although I don’t like facebook I acknowledge that many people hang there. It’s only 1 click to like and your post is displayed in one’s feed. Stumbleupon can also provide you lots of traffic in short time. And Google+ is nowaday trend. One-click plus may improve the ranking of your post in serps (although it’s hard to tell to which extent) and your visitors can at the same time share this post to their Google Plus account.
So I think among many social networks it’s better to focus on these 4. I believe people are lazy and it’s better not to give them to many variants for submission and stick to those 4.
Here is the link to this plugin Add Twitter Facebook Google Plus One Social Share