Likebaiting is the practice of trying to compel Facebook users to click the Like button associated with a piece of content. The practice is similar to linkbaiting, in which content producers craft content with the intent of getting people to link to it. A "Like" is an endorsement of a post, product, business or some other page content, registered by clicking the button associated with that item.
There are various approaches to likebaiting. The poster may simply ask viewers to "like" an image or ad and supply a reason to do so. A common example is an image of an antique item, such as a washboard, with the text "Click like if you know what this is." Similarly, some posts offer a reward for a "like," such as access to content or entry in a contest. Another ploy is to post a contentious statement and ask viewers to "like" if they agree. More organic methods of eliciting "likes" include posting pictures of celebrities, famous quotations or statements that are likely to be popular with many users.
Likebaiting isn't unethical per se -- if you post content, you naturally want it to be liked and shared. The typical Facebook user shares content that they find compelling and leaves it at that. However, a business gains visibility the more likes it can accrue and, as a result, some resort to deceptive or even fraudulent means of generating large numbers of likes.
Likejacking is a variation on clickjacking in which malicious coding is associated with a Like button. The most common purposes include identity theft and the dissemination of viruses, social spam and hoaxes. Unscrupulous businesses sometimes use likejacking methods for fraudulent promotion of their brand or a product.