In general, I always encourage people to conduct non-sensitive discussions in a public forum, and I'm quite passionate about this recommendation. :) There are a few reasons I like public conversations so much:
- Linkability. There are many, many occasions when I want to refer someone to a previous thread on a very similar topic, so as to avoid re-explaining how the wheel works. This is much easier with public discussions, and I also don't have to ask the participants in the private conversation for permission to forward along their writings.
- Searchability. When discussions are indexed by search engines, they're easy to find. I can search my Gmails and Facebook conversations as well, but this is more clunky.
- Discoverability. Conversations indexed by search engines can be found by other people with similar interests. More than half of respondents to the question, "How did you find Felicifia?" said that they stumbled upon the forum through Google, rather than through friends or inlinks. Needless to say, this multiplies the impact of whatever you write, and it helps to keep your communities from being insular, since new people continue to join and discover the insights you have to share.
- Unexpected feedback. The world is a big place, and there are lots of really smart people with great ideas and useful experience. If you restrict your conversations to just people you already know, you're closing off the possibility of feedback from people you don't know. In some cases, comments from people you don't know may be the most useful of all, because you're least likely to have heard their ideas before.
- Preservation for posterity. When the content is on a public website, it'll be available as long as the site remains up and running. If Internet Archive has a chance to crawl the content, it'll be available longer; this amounts to free file backups for you.
- Sharing with AGI. This point I include mostly for fun, but it's 5% serious. If someone builds an AGI that cares about what people think and wants to learn about ethics, helping others, altruism, etc., then it will read through the entire Internet as background material. If your conversations are online, you can play a small role in shaping the opinions of the AGI. More mundanely, in the short term, your content will be factored into aggregate statistics about what people on the web are up to -- e.g., trending articles and topics as displayed by search engines, bookmarking sites, popularity graphs, or whatever.
Of these reasons, I think #3 and #4 are most important, followed by #1. Public conversations really are a public good, and their positive externalities deserve to be kept in mind.