It has long been my habit in informal correspondence to use "hiya" as a salutation/greeting. I have always thought it to be very friendly, and a little less tepid than some of the alternatives. And "hiya" is on my mind at this moment because some folks here in Taiwan are only familiar with the more traditional forms of greeting; they are prone to ask, "What is the meaning of 'hiya'?"
The "Altavista revolution" has now made it very simple business to say something sort of quantitative about "hiya" and some of its alternatives. I have just made three little Usenet searches. It is reported that "hiya," "hi," and "hello" occur in Altavista's Usenet database with the following frequencies:
Needless to say, there are lots of other terms that can serve as informal greetings. And some hi's are not greetings at all. It's even possible that some of the hiya's and hello's are not greetings. But it looks pretty safe now to say that only about 0.4% of the Usenet greeting words are hiyas. It gives me comfort to be on the less traveled road.