Online Play is the only way that Tony Hawk games stay alive, in 2007 after a player by the name of biohacker hacked Neversoft's FTP server, they decided to removes the MOTD files (message of the day files) from tony hawk's proskater 4 to tony hawk's underground 2, American Wasteland not requiring that file continues to thrive to this day.
For a few years now, Members of the online community have been connecting through several different ways online, from motd replacements to a simply different lobby server, they've found a way to keep the game decently alive and continue to have tournaments, clans and such; below this article is a list of websites that help facilitate online play.
THMODS.COM - Tony hawk modifications, most of them automatically repairing the ability to go online
THPS.US - A website to get back online on PlayStation 2 Consoles
Clan websites also offer these same features and also more since they are the most active
PHUNONLINE.COM - Popular Playstation 2 Clan website
NG-THPS.COM - Popular PC clan site
Happy Gaming!