This is really just in response to a question on Twitter by Zabeth, but I didn't think I could fit my answer in 140 chars.
Over iamdiddy's last 200 tweets (as of about 15 minutes ago) there were 17,649 characters, for an average tweet length of 88.2 characters. Of those 17,649 characters, 708 were exclamation points (4% overall) and 137 were questions marks (just under 1%). The longest string of exclamation points was 10 and the longest string of question marks was 7.
On average, then, we would expect an average tweet by iamdiddy to have 3.5 exclamation points and 0.6 question marks.
(I could get a bigger sample if I was willing to go more than 10 pages back. But I'm not. Bedtime!)