Social bookmarking site Diigo announced earlier this week that it was changing its pricing model, offering premium, ad-free accounts with more features and better support. Diigo will still offer a free version, as well as a free version for educators, but both of these have some limitations.

In the free version, for example, you can only make 50 highlights per month. The number of bookmarks is unlimited for all versions. The premium version offers full-text search and the ability to do screen captures and annotations.

Diigo's move to the freemium business model follows on the heels of Ning's announcement in April that it was ditching its free version altogether. The decision to keep the free version and to address educators from the outset is a good move on the part of Diigo, which is likewise a very popular tool among educators.

The premium version will cost $40 per year.