They are all singular, in that we never pluralize them. The only one which makes sense as plural is "someone" and we use a different plural pronoun in that case - "some". They also all act as singular. With "someone" and "anyone" that's obvious, since they're referring to single people. However, "everyone" is also grammatically treated as a single object (the object of all people). For instance, Someone is happy. Someone was happy (is/was because singular). Some people are happy. Some people were happy. (are/were because plural). Everyone is happy. Everyone was happy (is/was because singular). As for the verb which follows, e.g. "tell"..Somebody tells.. Anyone tells.. Everyone tells.. It's not really about plural though, since: I tell. They tell. She tells. That person tells... Hope that helps! :)

yes... and even the collective noun "everybody", they are in 'singular from' and you can add "s" or "es" to a verb following it. :)

Personally, i think they should be used in singular form.