What are the effects of having a Facebook "Like!" button on your page, if you are a small site?
What are the benefits of it, and possible disadvantages of the Facebook Like button?
What effect does it have on promotion?
I'm curious about this too. I had a similar question here, with some nice answers: http://answers.onstartups.com/questions/12233/has-using-the-facebook-like-button-been-beneficial-for-your-business.
Using my relatively novice experience of the Facebook Like button, here's what I would say:
So far, overall, I think it's worth implementing and testing for a while. The disadvantages aren't significant (in my mind, at least) and the advantages could be great. I'd be curious about what others here have to say though. This is just from my meager experience thus far.
At a minimum, I would ensure you have a Facebook page for your business (previously known as a fan page). Then, you can add a "Like" button that links to your Facebook page. The advantage is that as you get more people as fans/friends of your page (and thereby your business), you're building your reach.
For an example, you can see how I do it at http://onstartups.com. I have two kinds of like buttons. One for the "site" (which connects to http://facebook.com/onstartups) and one for individual blog posts so that people can share them with your Facebook friends. Works pretty well.
Mike did an excellent job describing the advantages and disadvantages of the Facebook Like button. Here's an article that discusses 8 of Facebook's recent social plugins for your site, including the like button.
I still prefer the older share button over the new like button because I think the way the share button displays the information on someone's Facebook news feed makes it more clickable. The advantage of the like button over the share button is that the like button is a lot quicker to use for the reader, so it has the potential of being used more often than the share button.
My recommendation would be to use the share button on each post along with the Facebook Like Box (discussed in the article I linked to above) through out your site. Also, check out that article, you may discover another plugin that works for you.