There is a bot activity in Twitter and at the
moment is related to the new followers gaining only. What is
happening is “profile me” bot is exploring all Twitpic hosted
pictures replying to the authors with the same text phrase:

The bot started working on Friday, Dec 23 at 9 pm (GMT -05:00) with
the highest peak on Saturday, 3 am the same GMT zone with 0.19% of
all Twitter traffic.
In spite of the bot being used to gain followers and to promote
porno content via bio user information, potentially it could be used
for any other malicious purpose – like malware spreading via adding
additional short URLs to the twits.
We’re monitoring it.