Having these various goals, until recently I haven't really thought much about how to monetize these sites directly. But I've been trying a few things lately.
By far the best option seems to be to promote Amazon Associates products. There are just so many of them to choose from, and they are very easy to share on Twitter and Facebook in particular. That's because when you're logged in and have found something you want to promote, you can just click one of the little share buttons at the top of the screen and an update is created instantly for you.
I've been using this facility on Twitter quite a lot lately and the sales come trickling in. (I don't have many followers on Facebook yet, so I haven't tried it there yet.)
I think Amazon Associates goes particularly well on Twitter because there your network will tend be focused on a particular interest. So you can find ebooks and other products that they're quite likely to enjoy. (Facebook is less interest focused. It's more social in the truest sense of the word.)
I've only been promoting books so far. Maybe other kinds of products go well, too. I'll find that out in time ...
In any case, books are ideal. Firstly you can find really good quality ones for next to nothing. And they can often be downloaded instantly as Kindle versions. The low cost, immediate availability, and specificity of subject matter combine to ensure a good conversion rate.
Also, so many of the books have titles that can easily be converted to popular hashtags, increasing the likelihood of potential buyers finding them on Twitter. (If you are going to promote products this way, keep an eye out for this aspect.)
The other reason they go so well is that the book cover image appears automatically in your tweet. If it's appealing and eye-catching, it can brighten up your stream and increase the odds that your followers will click on the link.