The short answer is yes, the flash memory cells in a USB thumbdrive can be written to a limited number of times before they lose their ability to retain data correctly. The number of write operations a memory cell can handle over its lifetime generally ranges between 10,000 to 100,000, depending on the quality of the product. Once the limit is reached, stored data can become corrupted and totally lost.
It is a good idea to have a copy of your data on at least two different storage devices for safety and backup purposes. If you are accessing and working with a file on a usb flash drive, it is best to first copy the file to your computer’s internal hard drive, open it and work with it there, then after saving changes, copy the file back to the flash drive, overwriting the old version. Working with your files this way can greatly reduce the number of write operations to the flash drive and extend its life.
For a lengthier, more detailed answer to this question, check out the following article on Ask Leo!
~Ted Eiler