Is participating in professional meetings helpful from a CV point of view? My professional society (a physical sciences one), arrange an annual meeting where almost 10,000 people attend and almost one third of them present. The benefits of participating are overwhelming, but I was thinking whether or not such participation is useful on a CV, especially that what you present is not going to be published (although normally you present your work which is to be published or has just been published).

Note that this is a meeting, not a conference.

EDIT: by participating I mean presenting.
The key factors here are that it's a contributed presentation (not invited) and it's in a huge meeting with many presentations of differing quality.  It sounds like you may be talking about the APS March meeting, but similar things are true in many other cases, such as the AMS/MAA joint meetings in January.

How valuable such a presentation would be on your CV depends on how far along you are in your career.  If you are coming up for tenure, then you'd better have far more impressive things to list on your CV, and presentations like this would be inconsequential.  (The only value on your CV would be in demonstrating continuing scholarly engagement if your job is almost entirely focused on teaching.)  If you are a college sophomore, then presenting at a meeting like this would be noteworthy.

To a first approximation, if you're an undergraduate then these presentations are worth highlighting on your CV (unless you are lucky enough to have something more impressive to emphasize), while if you're a grad student they are worth listing but probably won't be very important.  However, it's worth asking a mentor for personalized advice.