One of the trending phrases on twitter lately has been 'True Blood' due to the popularity of the True Blood TV series. I've noticed lately that most trending terms in twitter have quite a large number of spam tweets and this is no exception. I've used Twitter Venn to try and get a feel for what the proportion of the spam tweets are for this topic. A quick glance at the search results showed large numbers of spam tweets mentioning free grocery money or gift certificates so I did a twitter Venn of 'True Blood' versus 'grocery'.
Based on the tweets at this time there are 8597 tweets/day for 'True Blood' that don't mention 'grocery' and 3781 that mention both. This gives us a spam proportion of approximately 3781 / (3781 + 8597) = 31% without even including spam that don't mention grocery. If you look at the red word cloud for 'True Blood' without 'grocery' you can see that there are several other spammy words that are fairly prominent - 'won', 'free', 'cash', 'gift', 'cards'. This suggests that the amount of spam for this topic is even higher.
These numbers do change quickly because they are based on the latest tweets only. To do an accurate analysis would require looking at more data over a greater period of time.