Broken Base: After SOCOM 2, which the fanbase collectively agrees was best SOCOM game, with the original not far behind in quality, the fanbase splintered in many different directions with the following titles (see Contested Sequel).
Contested Sequel - Averted with SOCOM 2, which received as good or better a reception as SOCOM 1, but the other sequels are more polarizing:
SOCOM 3 (and by extension, Combined Assault) - To multiplayer, Zipper added the ability to swim and modify your weapons, a day/night cycle, vehicles, optional spawn points, and created bigger levels to provide 32-player matches. Fans either embraced the features as welcome additions to the SOCOM franchise or rejected them as a poor man's Battlefield. With the single player, however, SOCOM 3 is widely considered the best one.
SOCOM 4 - Zipper finally released a proper sequel to SOCOM 3 onto the PS3 six years after SOCOM 3, though many of the single-player and multiplayer features were altered, alongside the general gunplay and controls. The old multiplayer lobby system was replaced by a matchmaking one, the older non-deathmatch modes were replaced by newer ones, and respawn settings (though still optional) were emphasized more compared to the older games, and campaign mode became more action-movie oriented than methodical and tactical. The fans who don't like the game loathe it for these reasons. The fans who do accept SOCOM 4 generally enjoy it, though with some reluctance because of its questionable design.
Seinfeld Is Unfunny: Once the crowning franchise for Sony's fledgling online service back in 2002, SOCOM has become all but irrelevant in the online shooter genre (for consoles, anyway). Thanks to too much time passing between major titles - six years went by between SOCOM 3 & 4 - many popular shooters filling the void SOCOM left behind, and an overall lackluster reception for SOCOM 4, even the die hard fans aren't finding much reason to stay loyal to the series.