Apologies for the late reply.
I'll clarify those emails and the whole issue.
A while back we introduced sign up through social networks like Facebook, Twitter, Google and VKontakte.
Due to their terms of service, we cannot keep any user information we obtained through their API in our databases for those accounts that have been inactive for 6 months or more.
This work was ready at the beginning of April 2014, so in October we started to mark inactive accounts (if originated via social networks) for deletion and emails were sent offering the possibility to reactivate the accounts.
The reactivation itself worked fine, but to complete the process a sign in following the reactivation was required.
Apparently many didn't sign in after clicking the reactivation link and seeing the "welcome back" page, thinking that at that point everything was done.
Yesterday we deployed a "fix" so that even without the sign in after the reactivation, the account is fully reactivated and won't come up as inactive the next day.
Sorry for the inconvenience.