You might be surprised. I think that it could be possible that AVSIM could apply for 501c3 status as an educational organization.
IRS Code defines a 501c3 as:
To be tax-exempt under section 501(c)(3) of the Internal Revenue Code, an organization must be organized and operated exclusively for exempt purposes set forth in section 501(c)(3), and none of its earnings may inure to any private shareholder or individual. In addition, it may not be an action organization, i.e., it may not attempt to influence legislation as a substantial part of its activities and it may not participate in any campaign activity for or against political candidates.
Organizations described in section 501(c)(3) are commonly referred to as charitable organizations. Organizations described in section 501(c)(3), other than testing for public safety organizations, are eligible to receive tax-deductible contributions in accordance with Code section 170.
The organization must not be organized or operated for the benefit of private interests, and no part of a section 501(c)(3) organization's net earnings may inure to the benefit of any private shareholder or individual. If the organization engages in an excess benefit transaction with a person having substantial influence over the organization, an excise tax may be imposed on the person and any organization managers agreeing to the transaction.
Section 501(c)(3) organizations are restricted in how much political and legislative (lobbying) activities they may conduct. For a detailed discussion, see Political and Lobbying Activities. For more information about lobbying activities by charities, see the article Lobbying Issues PDF; for more information about political activities of charities, see the FY-2002 CPE topic Election Year Issues PDF.
Exempt purposes are:
The exempt purposes set forth in section 501(c)(3) are charitable, religious, educational, scientific, literary, testing for public safety, fostering national or international amateur sports competition, and preventing cruelty to children or animals. The term charitable is used in its generally accepted legal sense and includes relief of the poor, the distressed, or the underprivileged; advancement of religion; advancement of education or science; erecting or maintaining public buildings, monuments, or works; lessening the burdens of government; lessening neighborhood tensions; eliminating prejudice and discrimination; defending human and civil rights secured by law; and combating community deterioration and juvenile delinquency.
Please keep in mind that none of the above means that AVSIM cannot have paid employees, most Non-profits have paid employees. This would mean that taxes would not need to be paid on the ad revenue or the donations you receive that go above and beyond the direct expenses related to operating the site.
**Legal Disclaimer: (I am not a Tax accountant or a tax attorney, please do not consider any of this as tax advice and seek the advice of a paid professional as relates to any tax matter. This is strictly my opinion based on some life experience and a little research)