I fully support all asynchronous, as well as a blended approach (and I do not mean f2f as the synchronous piece). I have used very successfully Chat for guest speakers to come in and share what they know about a topic. I have students generate 1 question in the course (so everyone can see the questions and there are no repeats). Then, I send the questions to the speaker and they answer the questions by including the question and their response on a word document.
When we have the Chat, there are rules to follow for communicating. Students arrive to the Chat a little early to practice the rules, which take about 1 minute to master. Then I facilitate the process by following the rules, which are here - Chat Rules. The speaker also knows the rules and as I type in one of the questions for the speaker, they respond by following the rules. The truly synchronous piece comes when I ask students if they have any questions after the original question has been asked. They raise their hands (according to the rules) and I facilitate this process. When there are no more questions (and I have to watch this, because the Chat is only 1.5 hours long), I move to the next question students posted in the course and repeate the process. I keep an archive of the Chat for those students who were unable to come to the Chat - which are never mandatory due to the synchronous nature in an otherwise asynchronous course.
I would like to know how others use a blended approach to their courses or what others have experienced in online courses that were blended in nature.