I created a temporary email service that was being used by about 10k users / week. Then several weeks ago, the number of users started growing like crazy up to about 60k users a day. Then we checked the recent email activity and 60k / 65k emails were from a social networking site.

Seems our service was being used to create fake bot accounts. The newly created accounts were obvious fakes. Rather than deal with the issue, we just shut the service off.