SAN FRANCISCO — Facebook, the world’s biggest social network, is selling more ad spots to big companies like Wal-Mart Stores, Procter & Gamble and PepsiCo.

But the site’s pages are also home to countless ads from smaller companies that can be funny, weird or just plain creepy — those suggesting you are, say, eligible to get a free iPad because you are exactly 26 years old, or entreaties to see what your offspring would look like if you had a child with a celebrity.

Odd Web ads, like the dancing women promoting mortgage brokers, are not new. But on social networks like Facebook, where people go to communicate with one another, advertisers seem to be trying especially hard to intrude on the conversation.

The so-called self-service ads on the site, from the likes of video game start-ups, herbal supplement makers, sweepstakes companies and wedding photographers, are shown on the right side of most pages. Many advertisers who use the self-service system are tempted to go as far as possible in making ads that attract attention and appear relevant, aided by the information that people give to Facebook.