Recently, Will Schmitt of Classy posted a blog discussing the limitations of text to give and text to donate services (he distinguishes the two). The blog does point out some interesting considerations, but it overstates the limitations of text to give.
[UPDATED] Legal Compliance in Text Message Marketing
When a prospective donor opts in to your keyword, by law you must inform them of the number of future text messages you plan to send them per month. You must also give them clear instructions for how to stop receiving future messages from your organization. We take care of this for you so you don't have to worry about being in compliance.