Irene wrote a good answer about the verbal case, but didn't mention writing. In formal writing, it is fine to write "Dear XXX" at the beginning of a letter. For XXX you can use a name, and/or a title, or if you don't know, you can use something like 'Sir' or 'Madam' if you know the person's gender. If you don't it's probably better to say "To whom it may concern" and avoid the 'Dear'. And just to add a difference, here in New Zealand, being addressed as 'Dear' by a waiter would be a little strange, unless it was an elderly woman.

Hi Vivian, What an interesting linguistic and cultural question! In General American English (so, outside of the US South or other regional subcultures in the US), I would say that "dear" is most commonly used in the following contexts: older people addressing younger people, married people addressing each other, and some servers in some types of restaurants talking to the customers they're serving. Husbands can call wives "dear" in English, and wives can call husbands "dear". But even that's sort of old-fashioned now. Many younger married couples have other "terms of endearment" for each other, like "honey" or "babe". (Same-sex couples may or may not use "dear" or other endearments.) "Dear" is a term that older *women*, in particular, may use with younger people regardless of gender. An older aunt who speaks English can call a nephew "dear" just as easily as she could call a niece that. Grandmas (grandmothers) also can use "dear", and maybe grandfathers (grandpas) can too, although that's much rarer. "What can I get you, dear?" could be one way a waitress asks her customers for their food orders in a diner. But no, I would not say that "dear" is as universally or casually used as "hi". If any stranger online called me "dear", I would feel they were trying to belittle me. It's not my cultural expectation, as an English speaking woman, to use "dear" to address female friends who are my age (or older or younger), either. I hope this helps!