Give a strong motive to your visitors to share your blog on Facebook, Twitter and Google Plus: lock your content until it is shared.
Social Share Motivator is a flexible and very efficient plugin that will help you increase the readership of your pages and get something back for all those posts you publish free of charge. It makes the content of your blog hardly visible (to visitors, not to search engines), urging them to share it on Facebook, Twitter or Google Plus, if they want full unhindered access. Since version 4.0 the plugin features responsive social icons for better display on cellphones and tablets.
Social Share Motivator can be (optionally) very aggressive by not including a "Close" button in the Share prompt, forcing your visitors to "share or leave" (although the Escape key still works).
After a visitor shares your page, all of your site becomes available. Activating the option of a "Close" button, you can let your visitors read your blog without sharing at this time (prompts on other pages won't stop, though)
Motivator can lock specific posts and / or pages, for example only your download page, leaving the rest of your site easily accessible as always.
Besides this, it can lock the whole site but excluding specific pages or posts. This way you won't have to shield your contact page, for example.
The plugin includes also options for custom page titles (letting you define the title and url of a page that will be shared instead of the currently visited), and options to customize the style of the Share prompt (font color, background color, etc). The plugin is compatible with the current versions of WordPress and also with multisite MS / MU installations.