A survey of 4300 networks, representing 140,000 direct exchanges of traffic, so called peerings, on the Internet, found that 99.5% of “peering agreements” were on a handshake basis, with no written contract and the exchange of data happening with no money changing hands. Moreover, in many locations, multilateral agreements are in place, using a so-called route server, where hundreds of networks will accept to exchange traffic for free with any network that joins the agreement. The parties to these agreements include not only Internet backbone, access, and content distribution networks, but also universities, NGOs, branches of government, individuals, businesses and enterprises of all sorts – a universality of the constituents of the Internet that extends far beyond the reach of any regulatory body’s influence.
Internet traffic exchange: 2 billion users and it

Of course, when counting by number of agreements, the smaller ones tend to dominate the statistics. Hence, that last 0.5% surely reflects a proportionally larger percentage of the actually traffic exchanged. Certainly most of the large backbones are more formal about their peering agreements.