Here's something that may help museums increase their online presence --- Google has a grants program that offers in-kind advertising through its AdWords product to selected grantees.
Here's a quote from Google: "Designed for 501(c)(3) non-profit organizations, Google Grants is a unique in-kind advertising program. It harnesses the power of our flagship advertising product, Google AdWords, to non-profits seeking to inform and engage their constituents online. Google Grants has awarded AdWords advertising to hundreds of non-profit groups whose missions range from animal welfare to literacy, from supporting homeless children to promoting HIV education."
You can find out more at this Google Grants website.