Twitter is a different matter; there are a bunch of different metrics out there (of which my favourite, despite its imperfections, is Crowdbooster) which enable you to see which of your tweets has been picked up by the Twitterverse at large.
My most retweeted tweet ever was on 7 November 2010, a link to a Livejournal entry:
My most frequently retweeted tweet of the last twelve months was on 5 December, as I livetweeted the International Court of Justice's ruling against Greece on the Macedonia name issue:
ICJ says that #Macedonia entitled to refer to itself as "Republic of MAcedonia" in dealings with Greece!!!! http://t.co/Aj65uyPUThis was picked up by 18 people, with (again according to Crowdbooster, which missed a couple of them) an outreach of over 6,000. The typo is a bit embarrassing, but there you go.
However, a single retweet by Paul Cornell, who has 16,000 followers, gave much more depth of penetration to this message on 16 August:
D'oh! If I had realised Randall Munroe writes xkcd, I'd have voted for him and he would have been joint winner in the #hugos. #renosfCrowdbooster claims that I got four replies to this (but I only seem to have records of three, from @niallharrison, @omegar24 and @elmyra).
I don't have any way of tracking the longest conversation I have been involved with on Twitter, and I think it would be quite difficult to compare, say, a prolonged back-and-forth with a friend which is mainly seen by the two of us, versus a broader exchange between lots of people to which I may have only contributed once or twice. No doubt there are mechanisms out there which will claim to quantify that sort of thing.
If I were more concerned about building my online profile I would now be planning all kinds of optimization strategies; but I am not!