Here is some information about the passenger traffic to the Scilly Isles
Souce:
UK Airport Statistics | Aviation Intelligence | About the CAA
From the same souce We also learn that ISC had 3,260 passengers in January 2011 and 16,603 in August 2011. In other words it's 5 times as many passengers in August compared with January. From this it's easy to estimate the base line for the local use and with that also know that around 68,000 of 107,000 passengers at ISC are tourist visiting the islands. I guess it would have been the same share of tourists on the flights to Tresco.
Since someone above talked about Aurigny and their Trislanders. Aurigny is French word for the island Alderney. Alderney have around half the number of passengers as the St Marys and Tresco combined. Alderney is not as tourist dependent as the Scilly Isles - the ratio between summer and winter is only 2.4x (there are a number of larger airports on the mainland that have the same ratio as Alderney, but non as high as St Marys and Tresco).
Still there a lot of similarities between Alderney and the Scilly Isles, like population 2,400 vs 2,200, almost equally remote from the closest airport, only one airline company serving (from November for the Scilly Isles) ...
But there are also a number of differences, like the the runway ar ACI is both longer than 800 metres and oriented according to the prevailing wind directions, it is possible to lengthen the existing runway with ease, lower ticket prices for flights compared with ISC, ...