Why RAM used so high?

1 [########********************************************100.0%]　　Tasks: 56, 15 thr; 7 running
2 [#############**************************************100.0%]　　Load average: 8.73 3.63 2.08
3 [########********************************************100.0%]　　Uptime: 12:48:33
4 [###########****************************************100.0%]
Mem[||||||||||||||||||||||||||||||||||||||||||||||3918/3951MB]
Swp[||||||||||||||||||||||||||||||||||||||||||||||6142/6142MB]

System was shocked when concurrent users with Authentication LDAP reached about 2000.

OS : SUSE Linux Enterprise Server 10 (x86_64),Intel(R) Xeon(R) CPU E7530 @ 1.87GHz X 4.

ejabberd version: 2.1.9

Could my server support over 5000 concurrent users? and how?

Thx.