I would hazard a guess that most providers want to get an email because that is the least disruptive way and allows you to give them all the screenng info that they need  so that they can proceed to set up a date.

Using a PM is probably less efficient and they may not see it for a while whereas most providers check their email many times a day.

Reading the providers entire site thoroughly is a very important step towards setting up an appointment.

Be sure to give them all the information that they ask for if you want to be taken seriously.

I would avoid the PM idea unless she tells you it is OK. I've seen dozens of ladies over the years and only one communicates with me through PMs. Currently Email seems to be the most popular means of first contact but, as mrfisher said, it's important to read the lady's site thoroughly and determine what means they are most comfortable with.

For initial contact, always follow the instructions on their website or ad.  For future contacts, they will usually tell you, most of mine are fine with a phone call after first visit.

I do set up visits sometimes by TER mail pm, but usually only after I have already seen the provider and we banter back and forth on pm anyway.

Twice, I have set up first visit on pm, but again, it was someone I had already "talked" to by pm because we were both active on our local board.

Bottom line- follow their stated preference.

What to do:
http://www.theeroticreview.com/discussion_boards/viewmsg.asp?MessageID=31956&boardID=33&page=2

What not to do:
http://www.theeroticreview.com/discussion_boards/viewmsg.asp?MessageID=31971&boardID=33&page=

The provider will state how to contact them in their ad.