This article explains what the Like button is used for. This button appears on most quote "Summary" pages.

By clicking the Like button next to the ticker, Yahoo! Finance will post this update on the recent activity section of your Facebook wall. You will also receive updates about that ticker on your Facebook wall.

When you click the Like button for the first time, you will be prompted to sign in to your Facebook account. Once you have signed in, the Like update will be posted to your Facebook wall. In addition, the counter next to that ticker's Like button will increase by one, indicating that an additional user has clicked it.

There is no limit to how many tickers you can like. Liking a ticker does not necessarily mean that you think it will go up. It simply gives you an easy way to follow the ticker and to let your Facebook friends know that you are watching it.

Clicking Like a second time will remove your subscription and you will not receive updates from Yahoo! Finance.