It’s been two-and-half years since online social media service StumbleUpon hit the eject button from eBay, its one-time corporate parent.

Since then, the company has grown its users and its staff. The San Francisco-based company now has 100 employees (25 percent of whom are former Googlers, the company says), up from 30 employees at the time of the eBay spin-off.

And the service, which lets users discover interesting Web content that has been flagged by friends and people with similar interests, now counts 20 million registered users, compared to 10 million about one year ago.

Now StumbleUpon is rolling out what it claims is its biggest post-eBay change yet, with the launch of a revamped website design and the introduction “channels.”

By offering specialized channels, StumbleUpon will allow users to fine-tune the stream of Web content that they see on the service.  A user’s StumbleUpon stream should become a little more relevant, founder and CEO Garrett Camp explained during a recent visit to the company’s SF headquarters.