Please bear with me here. As an NTP (Non-Tweeting Person) I don't really have a clear understanding of how things work with Twitter and could use some basic information.
We're making an effort to increase social sharing of our pages by adding Facebook, Google+ and Twitter buttons to a number of our pages. In order to make things easier, we're automatically populating the description fields with the content from each page's meta description tag. As I understand it, a tweet is limited to 140 characters. Our meta descriptions vary in length, but we generally try to keep them at 155 characters or less. That means that for a page with a description on the long side, we're probably going over the limit on the length for Twitter.
But according to Twitter's documentation on how to create a summary card, I'm not actually limited to 140 characters. Here's their description for the twitter:description property (emphasis added):
A description that concisely summarizes the content of the page, as appropriate for presentation within a Tweet. Do not re-use the title text as the description, or use this field to describe the general services provided by the website. Description text will be truncated at the word to 200 characters.
So do I have more leeway than I thought I did? How can a tweet that can't be more than 140 characters default to as much as 200?
And when I look at the preview that shows up when I click the Twitter button on one of our pages, I get a little more confused. For example, on a page that has a meta description (and consequently, a twitter:description) of 146 characters, what I get when I click the Twitter button isn't 146 characters. It's a total of 151 characters, 120 of which are selected. The remaining 31 are the URL of the page. Of the text preceding the URL, we've got most of the content from the meta description, but the second and fourth sentences (there are four sentences in the description) are gone, and our company name has been replaced by our Twitter account name. And there is a red "-2" next to the "sign in and tweet" button, apparently indicating that at 151 or 120 characters, depending on how much you count, I've got 2 characters too many. If I delete a couple of characters, the counter goes to 0 and then into positive territory as I delete more, so that's definitely what it's there for.
Looking at another page, we've got a very short description -- just 82 characters. When I click the button, the text that populates the tweet isn't the description at all. It's the twitter:title followed by the URL. What am I missing here?