Social Proof are the little clues that the internet gives you that other people have done the same thing you're considering.

Monophobia is the fear of being alone. All humans have it - most us of have it a lot. You don't want to be the only one using a product.

The best example (even though it isn't online) is McDonalds:


The sign reads "Billions and billions served" and although the food may not be the best for your health, you still go.

Some online examples are:

Basecamp



Mailchimp (again from before)


Depending on your industry and offering, you might also want product ratings, reviews and guarantees present to reduce purchase anxiety.

Customer testimonials and the logos of publications/partners where you've been featured are also key to building social trust.