You can do so in your journal if it is flagged for the market place or it may get removed. You can do so in groups that allow it, there aren't many. You can do so in your thought bubbles all you want. You cannot go to others' chatterboxes and leave messages about it. You can leave as many in your own as you would like, however only you and people who visit your page will see it. You can put it all over your profile with no problems.
A private message or PM is the on site email system. You can message your friends through your inbox or message anyone else by clicking the send a message button next to any screen name on the main site. In answers you first need to click the name and go to the transcript then to their profile to find it. You cannot send random PMs to people asking for votes, that is considered SPAM.
at 4:26 PM on Aug. 28, 2010