You Should Have a Public Marketing Site
Typically in a web application the actual "application" part is behind an authentication wall which is not accessible to the public (and not easy to share with regular social networking share-buttons), but there is also, most likely, a public "marketing" side of the web application which is for most intents and purposes a regular website that can be easily shared. This part of your website is the part to publicly promote, and this part of your website is a "content oriented" site (at least I hope it is) which will allow you to apply advice relevant to it (for example, basecamphq.com is different than yourcompany.basecamphq.com (the logged in version)).
Don't Get in Your Application's Way
Also, keep in mind that paying customers of a web service probably want to use the service more than they want to help promote it themselves. If I'm logged in and doing my thing, I want to focus on my stuff (the stuff I'm doing in the application) and not be distracted by marketing buttons that are only relevant to the application's owner (promoting the app doesn't enhance my personal experience in using the app). Let your application do what it does best, and put your marketing material where it can do the most good.
Share Public Pages
So, first of all, put your share buttons on the public marketing part of your site where the urls are actually accessible to the public.
Make Sense of Where You Put Stuff
Second, keep your website's hierarchies in mind when you're deciding where in the layout to place these things. For example, is the ability to share the url of a page on your website more or less important than the name of your website? how about the title of the page? how about the main menu? how about the sitemap in the footer? how about the copyright line? Find where in the hierarchy of elements on your pages it makes sense (with regards to your goals) to put these share-button elements and then design them into the layout with that balance in mind.
Different Positions Have Different Impacts on Usability
I'm not citing any specific locations as this is highly dependent on your specific goals, your hierarchy, and your design/layout. For example, if sharing a page is, for some reason, the most important thing anyone could possibly do with your page, then it might make sense to place it in the top left, with high-contrast, and bigger than the logo/site-name where people will see it first. But if it's a little less important than the main menu, and a little more important than the sitemap in the footer, maybe a good place for it would be at the bottom of the page's main content or just under the page's title.
Only you can decide what makes sense for your particular need.