It looks like Twitter is making a small, non-intrusive push to promote more interaction between members of the service.
Some users are seeing a new message appear atop their streams on Twitter.com. It notifies them that someone (a particular @person) is "back on Twitter."
"Welcome @person back to Twitter," it reads. "You friend @person is back on Twitter. Mention them in a tweet to welcome them back."
Inside the prompt is a quick link to compose a tweet that mentions said @person.
This appears to be a fairly new feature and was just spotted by Fred Wilson over on his AVC blog (via AllTwitter). It's clear that Twitter is looking to help spur engagement between users, especially ones that they fear may be flirting toward the dark side (or have just come back from it). You know, that "inactive user" label. Any social network is going to be constantly waging a war against inactive users, but Twitter, specifically, has seen its share of troubles in that department.
In December 2012, Twitter hit 200M monthly active users. But some reports have put the number of actual Twitter accounts at well over 500 million. Of course, some of this discrepancy can be attributed to duplicate, fake, and otherwise spam accounts. But some of it has to do with users who created an account, quit, and then never came back.
It makes sense that Twitter would try to rope users back to becoming regular daily or monthly users, and tapping their pals to welcome them back and get them engaged seems like a good plan.
It's unclear exactly what prompts Twitter to feature an account in the "Welcome Back" message. It could be that the user tweeted again for the first time in a long time - or it could be that they reactivated a stagnant or deactivated account. We've reached out to Twitter for clarification and will update this article accordingly.