Oh the wonders of German precision: party membership numbers down to the unit! It's impossible to get anything other than approximate and probably massaged numbers for French parties.

The Parti Socialiste hit a high point with Ségolène Royal's campaign, about 280,000 members, but has since lost probably 40% (good article in French) and stands at around 170,000.

Sarkozy's UMP, having chewed up and swallowed almost all the political right, makes great claims: nearly 400,000 members, according to its site. It seems generally agreed that it has over 300,000, and is clearly the biggest French party in terms of membership.