Have you ever thought of establishing a minimum fee? It seems there are many very small jobs around. Every job causes some minimum effort for handling and establishing the context, and this effort is not really covered by the per-word fee. A particular example for extra effort are the Twitter postings because of the character limits - often you have to rewrite/rethink the translation to meet the length restriction. I also check the Twitter account in order to get a feeling for the matter in question. On the other hand, the word count is low here and the payout is meager. A minimum fee of, say, $3 (standard) to $5 (pro) for any job would cover that somewhat.